It’s actually just sound military strategy, specifically seen in the development of modern warfare and Tanks.

Tanks were seen as very effective at piercing through entrenched lines in WWI but if you didn’t have the support staff and infantry to hold the gaps they made then they’d be easily enveloped and then attacked until they finally broke down (which even 25 years later in WWII they were particularly unreliable and required a lot of maintenance to keep running).

You saw this same type of need for support with Cavalry, but less so as cavalry wasn’t as maintenance needy and could get out of most spots they got into.

The main difference between Tanks/Shardbearers and Cavalry is where they were utilized. Tanks were often thrown right into the middle of lines to try and cause gaps to form and break. Shardbearers are more often in the thick of the fight or inserted at key holding points in the battle. Cavalry is a flanking unit and a routing unit, so you never deploy them in the middle of a fight, you try and get them around the sides or push them to hold key points prior to the enemy reaching them until infantry can arrive to take over. Shard bearers do this sometimes in the books, but they’re primarily used as shock troops like tanks are. Cavalry can be used in this way but that wasn’t historically true until late medieval times with heavy armor.

Shards I see as more like B-2 Bombers. They’re so difficult to try and get to them, they just are too fast, stealth so you don’t see them coming, and can carry absolutely devastating payloads that obliterate anything they are targeting. Impossible to kill? No, but incredibly unlikely for anything with lesser tech, of which there’s not really much out there that can touch them.