Every good DM likes to have options. You don't always have time or the desire to make a monster from scratch to throw against your players, so having lots of pre-made ones hanging around can drastically cut down the prep-time for your next session. Since 5th Edition D&D is still fairly new, there's not bunches of monsters just hanging around to use. Well, Kobold Press is looking to fix that with their Tome of Beasts up on Kickstarter.
The book is pretty big, containing more than 300 new monsters to bite, claw, stab, and cast all sorts of foul spells at your players. There's all types, from dragons, to demons, to beasts, to undead, and others. More than just a spattering of stats, the entries also give you lairs for the monsters as well. You could almost just open up to a random page and have an adventure made right there for you (ok, not quite, but close).
The campaign is more than 2x funded and working its way through stretch goals. It's set to run for another 26 days.