This is a very broad question since there have been multiple treaties of this nature throughout the span of history so I am going to assume that you are referring primarily to the inter-war naval treaties such as the 1921 Washington Naval Treaty and the 1930 London Naval Treaty as those are better known. Of course in general, the point of any of these treaties is to limit the building of new warships or the increasing their combat potential by incorporating new technologies or better weapons. This has generally been done to prevent arms races between nations and to preserve the current balance of naval power which was the case in the interwar period. I apologize in advance for my mostly anglo-centric answer as that is what I specialize in, for the other nations I have a less extensive familiarity with them.
Initially, Germany as the loser of the great war was stipulated as part of the treaty of Versailles that they were limited to only having six pre-dreadnought Battleships and six light cruisers. This was to prevent the German navy from being a threat to the British as their High Seas Fleet at full strength during the war had the capacity to challenge the British Grand Fleet in the North Sea. Thus the main point of the naval component of the treaty was to constrict the size of the German Navy so that they could not adequately challenge the Royal Navy which at the time was also facing cutbacks.
Following this in the interwar era, the Washington Naval Conference and the ensuing treaty in 1921 was the major event that limited the construction, displacement and armament of ships for the interwar era. For the British, this was agreed to out of necessity. The Great War had taken a heavy toll on the British economy making it economically impossible to maintain a large navy need for the two power standard under the Naval Act of 1889. The British populace was also suffering from war weariness due to the magnitude of the Great War and thus were against heavy military spending. As a result, the British reduced the requirements to a one power standard where the Royal Navy would only need the strength to fight the next largest navy as opposed to two of them.
This was contrasted by the Japanese and Italians who began to adopt more expansionist policies due to militarist and fascist governments taking power in the respective countries. This outburst of nationalism and desire to expand their territorial reach led to plans to drastically build up their naval forces as that was the primary tool for projecting power. The British who were unable to economically or politically sustain the arms race at the point in time opted to follow to the lead of the Americans to establish naval treaties limiting naval warships. The treaty itself created a limit of tonnage of Battleships in each navy which preserved the balance of power by limiting the size of each navy to their current relative strength. The UK and the US were given 525,000 tons, the Japanese 315,000 and the Italians and French given 160,000 tons. Similarly, aircraft carriers although an untested weapon at the time was allotted the tonnage of 135,000 tons for the UK and US with Japan only allotted 81,000 tons. Thus this limited the build up of capital ships that would cause an arms race, ensuring peace would at least be maintain for a limited time.
The treaty itself also placed weight and armament classifications for each ship class in order to prevent the construction of "super" ships that exceeded other ships of a similar classification in combat capabilities. This was also done in order to prevent circumventing the treaty by classifying ships incorrectly thus pushing them to be outside treaty restrictions. For instance, the Washington Naval Treaty limited Battleships to have a displacement of no greater then 35,000tons each and have a main battery for no larger then 14inch guns. The 1930 Naval Treaty essentially renewed the terms of the Washington treaty and further developed the second point of ship classification. Cruisers with guns of no greater then 6.1 inches were considered to be light cruisers and those with guns no greater then 8 inches being considered heavy cruisers provided they all conformed to the limitation of 10,000 tons. Thus this further refined the guidelines set by the Washington Naval Treaty and attempted to continue to prevent an arms race between the powers by limiting the size of the fleet and the combat potential of the warships.
The main reason behind the limitation of displacement of the warships was to better classify them and to limit their combat potential. In the case of Battleships, 35,000 tons was not adequate displacement for designers to build both a fast battleship that had armour capable of withstanding a 14 inch shell as the combined weight of machinery capable of making such speed with the weight of the armour and the armour itself would exceed the allotted displacement. Similarly the limitations on the caliber of the weapons on the ship was to ensure that ships were not rendered obsolete by fast increasing caliber sizes that their armour was not effective against thus prolonging the service life of older classes and preventing a race to upgun ships faster. This also presented an opportunity to the UK which did not have adequate resources to replace their entire battleship line to modernize older battleships/battlecrusiers such as the Hood, Queen Elizabeth, Warspite and the Valiant which cost less and still helped to maintain the fleet strength.
The multiple treaties ultimately were necessary as in the climate of interwar period with fascist on the rise, it was an attempt to constantly limit the arms race and reduce world tension to prevent another war from breaking out. However, the 1936 Second London Naval Treaty included an escalator clause as the Japanese, Italians and Germans were uncooperative and did not ratify the treaty thus rendering it largely useless as the allied powers used the clause to upgun their battleships to 16 inch guns.
In summary, there were multiple treaties to limit the size and armament of warships in this era so as to prevent an arms race from breaking out and potentially escalating to war.
Sources:
The Washington Conference, 1921-22: Naval Rivalry, East Asian Stability and the Road to Pearl Harbor - Erik Goldstein, John H. Maurer
Between Two Oceans: A Military History of Singapore - Malcolm H. Murfett, John Mikisc, Brian Farrell, Chiang Ming Shun