Is it imperative to shift to a higher gear when in a descent?
No, the behaviour you're observing doesn't mean there's anything wrong with your bike.  You're just travelling faster than you can pedal in a particular gear.

For a moment, let's forget about descending and suppose that you're travelling on a level road, so the only thing that's pushing you forward is your pedalling.  In principle, you can travel at any speed in any gear, as long as you can pedal fast enough.  In reality, of course, there's a limit to how fast you can spin the pedals: your legs can only move so fast.

Now, think about what happens if you stop pedalling while you're moving forwards: the bike starts to freewheel. In fact, this will happen whenever you're pedalling slower than the bike is moving. You can try this out. As you're cycling along, pedal slower for a moment, and you'll notice two things: first, you'll hear the clicks of the freewheel as the bike coasts; second, pedalling is very easy because all you're doing is turning the cranks, chainrings, chain and rear cassette. None of your effort is going into moving the bike forwards.

This is exactly what's happening while you're descending: you're not pedalling fast enough to match the actual speed of your bike, so you're still coasting and the pedals are very easy to turn.

This is a perfectly normal situation: there's nothing wrong with the bike and you don't have to do anything about it.  However, even if you're freewheeling down a descent, I would recommend that you change into a gear that's appropriate for the speed you're travelling at, or your highest gear if you don't have a gear high enough. At some point, you will want to pedal again, either because you slowed down (corner, traffic, lesser gradient, bottom of the hill, etc.) or just because you want to go faster. If you're in an appropriate gear, you can just pedal and drive the bike forward. If you're in too low a gear, the pedals will spin with no resistance, which can be a huge surprise that throws you off-balance and causes a crash. Correspondingly, when you do start to pedal again, always start somewhat tentatively to make sure your gear is appropriate.  It's when you push hard against no resistance that you're thrown off balance.