I'm relatively new to DnD in general and 5th edition is my first experience with Dungeons and Dragons. I am confused with some spell mechanics.
Spells usually state that they do damage, such as:
Burning Hands (Lv 1)
A creature takes 3d6 fire damage
Ray of Sickness (Lv 1)
On a hit, the target takes 2d8 poison damage
But some spells only dictate that the hit points are affected:
Sleep (Lv 1)
Roll 5d8; the total is how many hit points of creatures this spell can affect.
Color Spray (Lv 1)
Roll 6d10; the total is how many hit points of creatures this spell can effect. [sic]
In this case what would I do? Do I do 5d8 damage if I cast sleep or 6d10 damage if I cast color spray? If so, why on earth would I use any other Level 1 spell as these two would be exponentially better than any other D&D 5e level 1 spells?