I think there was an article late last year about the USAF swopping F-22's in Alaska for F-15's. Basically it's the Russians flying near US air space, so it seemed a waste to use the limited number of F-22's - it increases the wear and tear on them, so it made sense to use F-15's.
Similarly, you don't always need the stealthiest plane on the block for everything. Sometimes you need something which can go far, and go fast, and have presence. The F-35 can't do that. Obviously the F-15 won't be fully loaded for that sort of work.
And why couldn't you have a fast bomb truck after the F-35 has done it's wonderful 5th Gen razzamatazz? "Beast mode" is still only half of the F-15X's load. Let the F-35 rest, and reduce it's wear.