The Stuart light tank seems a singularly pointless vehicle, lightly armored with a puny gun. It doesn't seem to have been much faster than a Sherman, and apparently was actually shorter ranged. It was often relegated to reconnaissance duty, but the gun was so useless the British routinely removed the turret from their lend-lease Stuarts late in the war to reduce the weight and silhouette. I do see that it had some use in the Pacific, where poor Japanese anti-tank capability made its weak armor less important. But right until the end of the war, the United States was equipping 25% of its tank companies in Europe with light tanks, when all the other powers had abandoned the light tank concept. Why?
I wouldn't say it's a pointless vehicle, especially "with a puny gun". Let's compare the Light Tank M3 with its contemporaries.
Put into production in 1941, the tank had 38 mm of front armour. In a vacuum that might not sound like a lot, but recall that the Germans were only starting to equip their medium tanks with 50 mm of front armour around this time. German requirements for fast reconnaissance tanks, the closest comparable thing to an M3, were for 30 mm of front armour. The Crusader, a British cruiser tank that arguably filled the role of a medium tank had 40 mm of front armour, so about the same. For its time, this was a very respectable amount of armour to have on a light tank.
Now for the cannon. Out of 37 mm class guns, the American M6 was the most powerful. It could punch through the front of a German 50 mm plate, which meant it could engage German medium tanks from the front (albeit at a short range). The side armour of newest German tanks, equivalent to the front of the older models, was still vulnerable from a large distance out (1800 yards by British findings). British data also shows that they could penetrated the 62 mm layered armour of a PzIV from 200 yards and 50 mm armour of a PzIII from 500 yards. The gun was anything but "puny" and continued to be relevant for a while after it was introduced. Recall that despite all its upgrades, the PzIV never had more than 50 mm of front turret armour, theoretically making it vulnerable to the M3 until the end of the war. Pretty good for a light tank from 1941, I would say.
Now for the turretless M3, aka "Stuart Recce". As the name implies, the tank was used for reconnaissance only. The lack of a turret made the tank faster and less noticeable, as you mentioned, and gave better observation. The British were no strangers to open topped lightly armed vehicles, and the task of a recon tank is to report the enemy's location and get out of there as fast as possible. Standard armament for German reconnaissance tanks was a 20 mm autocannon, which was not that much more powerful than a .50 cal machinegun.
Certainly, the tank was not any more powerful than a medium tank, but being relevant as a reconnaissance vehicle was better than other light tanks have done. For instance, the Soviet T-70, also developed in 1941, was much slower and less useful as a reconnaissance tank, while being at best as well armed as the M3. The Germans toyed with the idea of dedicated reconnaissance tanks, or at the very least faster versions of the PzII, but aside from a small series of Luchs tanks ended up producing the same old PzII until 1942, which was also of limited value as a reconnaissance tank. A fair number of these tanks were still kicking around on the front lines until 1944, despite being far inferior to the American tank in every respect.
In short, it's unfair to call the Stuart tank family pointless. The Medium Tank M3 was extremely competitive in its class when it was first produced, and remained useful on the battlefield long after all of its contemporaries were no longer of any use. As a light tank and then a reconnaissance vehicle, the Stuart did an excellent job.
Sources:
http://tankarchives.blogspot.com/2016/07/light-tank-m3-americas-first-thousand.html
http://tankarchives.blogspot.com/2018/07/reconnaissance-cats.html
http://tankarchives.blogspot.com/2013/04/american-guns-vs-german-tanks.html
http://tankarchives.blogspot.com/2017/10/light-tank-m5-peak-of-evolution.html
National Archives of Canada RG 24 C-2 vol. 12101 1/AFV 1 (L)/1 slide 906