The Bolter is not a precision weapon. If it was, it wouldn't have the necessity for an explosive round. It can be used as one, don't get me wrong, but the warhead means it can be used as an area of effect weapon, similar to how the .50 cal or Minimi/GPMG are used today.
They are used against targets in hard cover like rocks or buildings, not because they can punch through the rocks (although the 50.cal with time can probably chew them apart), but because they cause spalling and fragmenting of ammunition. If you've ever thrown something like shale or flint, and you've seen it shatter, then that's what makes it lethal, as well as ricochets, in addition to fragmenting rounds scattering pieces of metal everywhere. Put an explosive charge behind, either on a impact triggered timed delay like the typical bolter ammunition, or airbursting, similar to Metal Storm Ammunition, you've got a recipe for mincemeat.
Fired on full auto, even if the Space Marines cannot directly target their opponents, but know of their position, they can effectively spray and pray.
It's a brief marketing demonstration of its effectiveness, and what is mostly apparent about the XM25's HEAB (high explosive airburst) is that it has an aft-charge, causing fragmentation to targets it has already passed, which means it can do its signature firing into a room and making gibs out of whatever is in there - that's probably the metal storm ammunition type, which going by Codex:Space Marines is limited to "Sternguard" Veterans or "Seeker" Veteran Squads in Legion Crusade List.
Considering the nature of the operations that Astartes undertake, I think that's more of a gameplay balancing factor, and that the Astartes given enough time and information are probably able to requisition more ammunition of this type. I think the only reason the Boltgun ammunition isn't a direct quote of the XM25 HEAB is because a load of geeks in a shed in the black country or the early 80's weren't aware of its existence and their knowledge of weaponry wasn't as well known or availability to research (short of Jane's) was harder to get hold of for up and coming writers.
Edit - in addition to KF, my webbing on deliberate ops took 8 30 round magazines (only loaded 29 though, left the spring springy and reduced stoppages) in addition to the one loaded in my rifle, and I had a daysack which contained a further 300 rounds in magazines and either a 420rnd ammo can or enough for two reloads of the attached fire support group, whether it was the 60mm mortar rounds, spare LASM's, or up to 400 link of either 7.62 or 5.56. Throw in a couple of reloads for the Sig, and two full tracer magazines, and 4-5 grenades, and multiply that between 8 of us. That was a typical load for a deliberate helo-borne operation - fly in, drop between 1-5 clicks, yomp in, drop day sack, fire and maneuvre in, complete objective, and then yomp over to the helo for pick up. The amount of ammunition there was in case the operation went tits up, such as a Chinook or Merlin getting shot down, or there was a redirect to a TiC with T1 casualties en route to pick us up. I've only ever needed that amount of ammunition once, and that was when we pushed too far north nearly into Ghor to avoid an encirclement.
But consider that with Space Marine operations - for example Beta Anph IV saw a Terminator Squad infiltrate a genestealer infested network of prefabs - their ammunition soon began to run dry, and Culln lost 2 to genestealers. It doesn't state how much ammunition was present (and they were using Storm Bolters with obvious full automatic), but simply that there was probably only 6 seconds of ammunition available to the Assault Cannon - which you can assume is roughly analogous to something like a 30mm Avenger (the 7.62 probably was more like the rotor cannon of 30K Legiones), which is around 4200 rpm at full speed - the hopper for the Ass Can was likely around 400.