You will need optimizer tools like PC optimizer badly (to be used everyday, that is) if:-
You use your computer anywhere more than two hours a day, either for your online or your offline work.
You have to download files from the Internet regularly.
You visit more than ten websites in a single day.
You want to regularly install and uninstall software from your PC.
Your computer does not have, or did not have, a firewall.
Your computer has embedded spyware programs.

Running an optimizer scan on your computer and then deleting the unwanted files is quite an easy job, which happens automatically even as you work. Do not neglect on it, unless you want your computer to slow down interminably within a few weeks of its continued usage.