A lot of times, you are told to uninstall and reinstall a program if
it's acting up. Examples are Firefox, Flash etc.
Does this help because some of the program's support files might be
damaged due to malware/faulty disks? Is that the main reason?
If a program was working and then does not work, uninstalling and
reinstalling may fix it temporarily, but you still have the problem of
what caused it to stop working in the first place that remains.
I have never had to reinstall Firefox or Flash (your example) on any
computer to fix a problem - there is always some other problem that
needs to be fixed or the user does not understand how to use the
application properly. This is frequently perceived as an application
problem when in fact it is a user problem.
It is possible that malware or a user can screw up so many options in
an application that they can't be undone effectively so sometimes the
best option is to reinstall to get a fresh copy, but it would be the
user that caused the problem, not the application.
The person that tells you to uninstall, reinstall to fix a problem may
mean that they have no idea what the problem is or how to fix it and
needs to learn some better troubleshooting and fault isolation
skills. It does buys time, appears to work for a while (what a hero)
until the real problem comes back again later.
Since you did not define what "acting up" means (28,300,000 Google
hits for "acting up" and 325,000 hits for "Firefox acting up") it is
impossible to speculate on your particular issue.