For example Dead Space is split into multiple installers, anyone know why?

Keeping up with file size limits of yesteryear (FAT32), probably. Also makes it easier to download in case your connection isn't perfect and randomly corrupts things.

Multiple parts, not multiple installers. Explanation: see above.

If you're seeing an .exe, usually a small file, and a number of much larger .bin files, normally limited to 4 Gb in size, it's what clarry already said. But if you've been around since 2013, I doubt this is news to you, so maybe you saw a game with multiple actual installers, as in the .exe files? In that case, I'm thinking it has 32-bit and 64-bit versions, and you should choose which you want.

The EXE calls to the .bin files which are chunked parts of a whole. sometimes you cant download the full file and only a portion. It saves space and bandwidth and its also a tricky way to break part a game and shrink it. Say all HIGHLY compressible assets are in 23 folders. You create the structure put them in archive 3 and turn 2GB to 1.4GB. Its pretty ingenious done right. Ive seen 124GB games shrunk as small as 22GB before due to really well compressed assets.

Thanks. So when you download all 4, I click the exe? Does it know the other bin files are there automatically? How does that work?

Yes the EXE will only work if all the .bin files exist. They are a split apart archive and all need to be there for the game to install.

As post 8 says, and also they all need to be in the same folder to work, and the names(the bit before the dot) need to stay the same.