When should pick a melee support like Tidehunter?

 

Melee supports are usually known for their gamebreaking ults and/or great abilities (Ravage,  Guardian Angel, Epicenter,  Living Armor, etc.) but lack good laning stages. Melee supports would work best in a trilane when he/she can be the 4 position, but that's not going to be available much in pubs

You can't harass the enemy out of lane

Being a melee support does put you at a disadvantage because unlike a Crystal Maiden or Witch Doctor, you can't poke the enemy out of lane. In fact, if they are ranged then you are going to be taking hits all day long. Because of this, it is recommended to not be the only support on your team, and it's even better if you're not the only support in lane. Putting a Tidehunter or a Sand King with a Crystal Maiden is a devastating combo because of the set up and burst between the two heroes.

Because of the aforementioned reasons laning the melee support is harder

The worst case scenario is having a fully melee lane against a fully ranged lane; it is very difficult to gain an advantage when you have no push back. Having some range is good, though if you're a full melee vs melee lane then that also works well.

The most effective way to play a melee support is to be aggressive

You're not going to win a long drawn out poking fight in lane, so what you should do is look to be very bursty and try to go for kills (this is also why the ranged + melee support combo works so well). Tidehunter has Gush, Treant Protector has  Leech Seed, Earthshaker has Fissure etc etc. Use these to set up kills and gain an advantage that way