How to use reapers if not to destroy buildings?

Reaper are neither a meaty nor a support unit, meaning they are not good in an army composition. This is because of their cost and their build time. Reapers are good scouts and harassment units.

There are a few timing attacks that are nasty, but actually mass Reaper is not viable. So Reaper are reduced to:

• Scouting: especially with speed they are a cost effective scout compared to scans or slow units like Marine / SCV that get killed all the time.
• Harassing: Having a few Reaper that regularly visit the opponents mineral line can be compared to drops. The opponent has to stay in his base or he risks loosing his economy.

But you also need a real army, that can finish you opponent.

In HotS the Reaper has exactly the same role. Their building damage is reduced, but they get an out of combat healing ability, so they can harass more often without dying.

They're extremely strong harass units early in the game as they're able to kill Marines, Hellions, Zerglings, Zealots, Sentries, and workers with ease. They can thus be used to achieve map control in TvZ and TvT.

I've personally developed a TvT build that hits with a timing of 3 Reapers that also protects a quick expansion. While the 3 Reapers are completely useless against an opponent's Marauder or Tank, these are uncommon at that point in the game. I created a Liquipedia page about the build here that also has a few replays, if you're interested. The timings are pretty tight, though.

In HotS they're being buffed with an upgradable regeneration, but I believe they're also increasing Reaper range. Either way, this will allow them to whittle down Queens over time and just generally be annoying, as long as you don't lose them.