If you do large water changes, which I do, your water has less time to allow the water pH to fluctuate wildly. The test kit is just giving a general range. Some fish, thinking mostly South American fish, prefer low hardness and lower/mid pH ranges. If a test kit gives a range and you are outside that range then they have an opportunity to sell you some buffer agent. Our freshwater fish come from a large range of environments, your ideal KH and GH will be determined by the fish you keep.
Typically you should try to avoid "adjusting" your water, most people end up chasing a moving target, too high -> too low (could be pH, GH, or KH), in the end the fish suffer. More advanced aquarists will adjust their RO water in some cases but for most of us it is not a good idea.
I haven't seen what your current parameters are, Angels prefer neutral to slightly acidic water, with lower KH and GH.