Is it necessary to clear variables before writing the same variable (updated contents) to workspace?

Kind of a silly question about a very fundamental thing. I forgot if Matlab always completely overwrites a variable if I reassign it. Or is it possible that some old content will remain if the old variable had more fields, elements, etc? More specifically, I have a GUI where one of the buttons is to send a couple structures (named "S" and "S2") to the base workspace in case I want to play around with them.

If I already had S and S2 in the base workspace from before, is there any need or reason to clear them in the base workspace prior to executing this line

assignin('base','S','S')

[Image Analyst]

Try it and see. By the way, sending structures to the base workspace is probably not a good idea. I never do this, and most programmers probably don't either. It's not needed for well written code.

[Alexei M]

Why not a good idea?