Monks: how to utilize them?

Hi folks,

I've been playing the game for almost 20 years, but only recently I've started playing ''seriously'' (I've spent the last year playing fairly competitive Starcraft 2).

I understand the role of most of the units in this game except monks... I mean, they cost a lot of gold, their movement speed is slow, converting units feel akward...

My question: what's their use in competitive play? How can I use them effectively? I feel like 100% of monastery technologies are absolutely useless.

Do they counter specific unit spams? Do I have to mix them with specific units to be effective?

Thanks for your time,