it also doesn’t tell you whether or not a person will vote for him
It doesn’t tell you whether a person will vote against them, but its a classic litmus test for turnout in favor. Below 50% approval rating signals a higher than average likelihood of an incumbent losing one’s seat. And the only countervailing narrative is when the primary opposition also has low approval.
Obama beating Romney in '12 was a case of two Presidential candidates being underwater and Romney simply having worse negatives than Obama on election day. Both parties saw a drop-off in voter participation relative to the '08 high water mark.