I guess the dictionary definition of the usage is the following:
どちらかというとその傾向であることをいう語。たぐい。 （＃4 精選版 日本国語大辞典）
So the sentences in the question literally mean:
- Mr Sato, do you tend to be good at English?
- This tends to be used often.
The usage may have derived from the original meaning of direction or side so that Xするほうだ can be understood as on the side of X.
The basic added nuance is if forced to choose X or not X, then one can say X is true (of the subject). So if making the comparison explicit, the sentences are something like
Another (though similar) way to look at ほうだ should be a weakening of the statement.
For example, as often said, the Japanese have a strict idea of command in a language, so
- 私は英語が得意です sounds like I can use English almost as a native speaker.
- 私は英語が得意なほうです sounds more modest like I think I can say I'm good at English.
- 酒は強いほうだ Generally I can drink a lot
- 私は酒にはつよい would mean capability of drinking like Russians.
Note ほうだ can more simply mean a comparative.
- 彼はクラスでは背の高いほうだ He is taller than average in the class.
The above can be considered a version of this usage where the comparison is implicit or vague.