Why do people bother with zipping files before emailing them even if they aren't very large?

Because emails travel via several computer systems each step of which could corrupt or lose an attached file. A zipped file is more likely to remain intact. By successfully unzipping the file the recipient knows that it is complete and has arrived as the sender intended.

Is it possible to send unzipped files via emails? Doesn't emails automatically zip them? Or maybe computers automatically down load them as zipped?

Zipping a file makes it harder to attach a virus to the file you can zip it and lock it with a password makeing it pretty difficult to alter.

The cryptowall virus uses a similar technique it zips your files and passwords them with encryption and demands a Ransom.

Often because they do not understand why they ever need to zip files anyway.