"Well, I only need the mobility required for XYZ (and no more than that)." Well, maybe, but maybe not. Your immobilities and weaknesses may be inhibiting your performance in ways that are not readily apparent simply by performing your regular activities. Trust me, it's no fun to only become aware of weaknesses after you've suffered numerous losses and injuries because of them.
This is Gray Cook talking about the need for hip and ankle mobility and stability in golfers who do not demonstrate the deep squat pattern in their regular training or competition. It's worth a look: