The current set of Polish borders are significantly different from the historical set of borders constituting what is "Poland". Poland existed as an independent state until 1795, when it was partitioned between Austria, Prussia, and Russia. Poland was revived as an independent state in the aftermath of WW1 but was then partitioned again by the Nazis and the Soviets under the terms of the Molotov-Ribbentrop pact, Stalin then annexed the eastern parts of the country into the USSR itself.

He then chose to "shift" the entire country westwards as the allies won WW2 against Germany, by giving it territory from Germany which had never being Polish before, this was followed by mass ethnic cleansing of the German population and the renaming of cities such as Stettin/Szczecin to help erase its German past and make it part of Poland.

People started to do it naturaly in 1945, but there were conflicts and misunderstandings. Commision was formed in 1946 http://en.wikipedia.org/wiki/Commission_for_the_Determination_of_Place_Names

It dealt with the biggest towns and cities first, and it finished around 1950.

As to why - the same reason English call Warszawa - Warsaw, Москва - Moscow, and Kraków - Cracow. Because it's more convenient when you can pronounce a place that you need to pronounce often.

Also propaganda (it was called "regained territories", and USSR wanted Poles to be always conflicted with west Germany, so they did everything to prove "Polish character" of these places, no matter that many parts of it last belonged to Poland in XVIIIth century).