Yes, they are needed. Role/auxiliary players need to be replaced internally , and you do that by insuring you draft well. The worst mistake contending teams make is either overpaying role players or trading for them externally and wasting draft capital. Teams do this because they don't have prospects ready to fill holes.
If we use Sandin as an example , you can consider the impact he has on the organization already. The Leafs may not be able to keep Gardiner (just the truth) if their young stars command heavy dollar. If this is the case, then presumably Dermott fills the hole. By doing that, Sandin suddenly becomes a vital piece within the next 2-4 years.
If you just punt 1st round picks for rentals , then you leave yourself devoid of options. When you don't have options , you do dumb things- like overpay for marginal talent.
Again, I'm not anti-trading picks , but it has to be for the right piece. That piece is pretty obvious in regards to the Leafs. There's absolutely no reason to harm your future flexibility because you want to upgrade your third line winger position for a couple months. A move that likely won't move the needle at all and will have future ramifications - even if they're unforeseen.