What use for? For big projects and awesome minds. For providing large art design, huge playground and unlimited imagination space. Examples: Big Zoo design. Computer. Gas factory. etc.
Will it break saves? Maybe, but we can avoid that through few tricks. We can add few bytes in the save file header to declare the resolution of this save, if it doesn't exist, set to default. Thus there's nothing different for old saves, they are merely saves with same resolution. For new saves, you can simply drag the window to reset the simulation area or type in the exact pixel value in option menu. Maybe we need scrollbars, for those saves bigger than a whole screen.
Big saves may causes extreme lag! Yes, but only if you add too much things. A default save with massive content is much more lag than a 3000x2000 save with few simple tesla instruments. Big drawing area is not a must, but a possible support for future designs.
First off, this has been suggested a lot before and will get rejected again. Secondly, it isn't only the particles that would make lag in TPT, it would be the screen its self. There is a reason why games like TPT have such small screens, It takes a lot of memory to make a screen the size of a large window. TPT would have to run more calculations to determine where particles will go or what they will do next; etc. Maybe when TPT gets recoded in C++, it will get a larger screen.