The HE grenade is the one most players buy last and throw worst. It almost never kills on its own, which is exactly why the throws that are worth learning are specific ones rather than a general habit of lobbing it at a site.
A direct hit at the centre of the blast on an unarmoured player does close to 98 damage — a kill in every practical sense, and something that essentially never happens against anyone who bought armour. Kevlar roughly halves it, and damage falls off quickly with distance and is blocked by geometry. Against a full-buy opponent, a good HE is worth somewhere in the region of a third of their health, not a kill.
Throwing an HE into a site at the start of a round on the chance somebody is standing there costs $300 and tells the defenders the round is starting on that site. It is the single most common waste of utility below the top ranks. If you cannot name the player you expect to hit and roughly where they are, the grenade is better unthrown — or better unbought, and the money spent on a flash and a smoke instead.
A player caught in fire and then hit by an HE dies from a combination neither grenade manages alone, and the fire is what stops them leaving the blast radius. This is a two-player play rather than a lineup, but it is worth knowing when your teammate calls that they have molotoved a corner.
Take a common CT setup where two players hold the same corner of a site. An HE landing between them might take forty from each — not close to a kill, and easy to dismiss. But it changes both of those duels from even to favourable, and it does so before anyone has peeked. Eighty damage spread across two players who now have to think about whether to reposition is a better $300 than eighty damage on one player who then heals to full next round.
This is also why the HE lineups worth learning are specific to a map's standard setups. A general "throw it at the site" habit produces nothing; a throw that lands exactly on the two positions your opponents actually use produces a numbers advantage every time they use them.
An HE thrown at a player defusing the bomb interrupts the defuse and usually kills, because a defusing player is stationary, committed, and frequently already damaged. If you are dead and watching, or pinned in a position you cannot leave, this is the throw that wins rounds you had written off. It is worth knowing the HE lineup onto each bomb site from wherever you tend to end up post-plant.
On a full buy with money to spare, yes — it fills a slot you would otherwise leave empty. On a tight buy, no. The four-grenade cap means an HE takes the place of a smoke or a molotov, and both of those change what is possible on the map while the HE mostly changes a health bar. Buy the HE when you have already bought everything that takes space, not instead of it.