Board Thread:Game Discussion/@comment-35520790-20180611041642/@comment-28169398-20180611051129

Use it if your career progression is going to get blocked if you don't.

This may also mean that you want to look ahead. You may not need the car in this particular series to advance, but it may be the introductory level car for a later series and if you have it, then you can unlock the showcase for the car(s) needed in that series to advance.