An eggplant can be baked or grilled; then you peel it. I read that you have to drain it then (e.g. in a colander), even overnight. What if you don't drain the water-does the taste change considerably? Can you still make spread out of it?
This is really about water content, not flavor. Eggplant contains a lot of water, and there can be plenty left after baking or grilling. If you then make a mashed/pureed spread like this, that water could make it a lot more liquid than you want.
That said, if you really thoroughly roast or grill the eggplant, you can get enough of the water out of it that you don't really need to drain it. If you prick a couple small holes in it, and bake for something like 45 minutes to an hour at 400-450°F (200-230°C), or similarly grill until it's well-charred, it'll probably be dry enough.
The exact details depend on the size of the eggplant, and what you're trying to make out of it.
Draining the water also helps getting rid of a somewhat bitter taste eggplants may have, especially if they are overripe. If you start with a fresh, young eggplant which is relatively light to its size (that's the best way to pick eggplants), then there's no need to bother with draining. The heavier eggplants may need draining, depending on their type and condition.