Why append-to-buffer function is written like that:

>(interactive > >(list (read-buffer "Append to buffer: " (other-buffer (current-buffer) t)) > > (region-beginning) (region-end))) ​ Is >(other-buffer (current-buffer) t)) part necessary? ​ Why it can't be >(list (read-buffer "Append to buffer: ") > >(region-beginning) (region-end))) alone?

That would behave differently. Read the docs to `read-buffer` and do some simple testing if you don't understand how it's different. (In general, you can answer most questions like this for yourself by reading the docs.)

You can also evaluate the change you propose and see what difference it makes.