Sometimes we're hungry for ideas. You kno... those days that you can't make a beat for shit, and then you're in the car later, hearin some mainstream shit, feelin a nice vibe off it, like "Why couldn't I do somethin like that today?"
Sometimes when there's no melodies bouncin' around your head you gotta just sit back and listen to some other shit for inspiration. Sometimes that's all it takes and you're golden, with something completely original just after giving your ears a listen of something different from your instruments (your blank white paper STARING UP at you).
Other times, you're in DIRE NEED to make something new; change your style real quick, or find what's really essential in someone else's beat. I've taken a basic melody from someone (not the exact thing, just the basic note they were in) and started with that. You'll be surprised at how many people have used those SAME EXACT notes with different rhythms!!! This would usually get me going if I have absolutely NOTHING to work with and my ears are tired.
Sometimes we tend to keep our "signature" beats the same way. I know cats who have a whole catalog of beats that sound exactly the same from each other, and that's fine too. I used to tend to hit the dark notes back in 94 and make those Rza/Mobb Deep beats to spit some heartfelt words on. When trying to make it "happier" it would sound too happy when not doing it right. But sometimes if you follow someone else's structure for a minute, and go from there, you end up with something COMPLETELY different, and YOUR OWN. You may even delete the first shit you played, and keep the inspiration from it as your actual beat.