A good filter is as much an investment as the lens itself ... I always sell a lens without the filter and keep it for continued use on another lens.
Ditto. Even when I didn't think I'd need it, as was the case for the 58mm filter I had on the 85/1.8…then I bought an MP-E 65mm. OTOH, I do have a couple of 77mm UV filters currently unused, since Canon seems to be releasing new versions of lenses with 82mm filter threads instead of the smaller ones of their predecessors.
A good filter will probably save the front element in an accident if no hood fitted