....the reason is that they don't address the cause of the algae in the first place and can possably make things worse (theres an influx of decaying matter - the algae - which puts load on the filter, more nitrates) - though not too sure about that
I think some algae can look OK - I never clean back and side glass. When I had my algae problem I tried the natural methods, split lighting period (4hr min gap), ottos, shrimps and sae's and now things seem quite balanced
Imho try other methods before resorting to chemical fixes (they rarely last the long term )