The Playstation 3 was released after the PS2 and people were excited to see it. The PS3 is obviously wider as compared to the slim version. It is also backward compatible which makes it the choice of a lot of people who would still like to play some games that may not be available when you have the slim version anymore. If you would like to revisit some of your PS1 or PS2 games, the usual PS3 will be sufficient for your needs. The PS3 slim, on the other hand, will have a slimmer build. It will also come with an upgradeable hard drive. This is going to be smaller and lighter which can make it more ideal when you need something that you can take from one place to another.
Like with all of the other consoles that were created before by Sony, When Playstation 3 was released, they have decided to create another version of it. The slim version is obviously slimmer, but there are some people who actually prefer the regular version of the PS3 mainly because of the backward compatibility. Aside from this feature and the fact that PS3 slim is slimmer, there is no more obvious difference between the two. People can just choose the variety which they think will fit them best. Some would prefer the slim version simply because it does not take up as much space as compared to the non-slim version.