I know what you're thinking: Clawitzer has no real justification for Download. That's true, it's just one of those cases where a generally applicable ability has very specific flavor. That being said, I think the best reasoning I can come up with it is that Clawitzer can both fire with its cannon and crush with its claw; it's very versatile.
Competitively, Mega Clawitzer functions similarly yet very differently from its base form. When it gets the Special Attack boost, its pulse moves will do damage similar to Mega Launcher on the pre mega version, but it won't be able to outdamage Specs sets. Meanwhile, upon getting an Attack buff instead, Mega Clawitzer can use powerful physical moves, but will do less damage with pulse moves. This allows you to intentionally not Mega Evolve when you want to click a pulse move and you know you're not going to get the Special Attack boost.
Although it already had the movepool for a Download Pokemon (STAB pivoting in Flip Turn, good coverage on both sides, etc), I personally chose to add Shell Side Arm and Stone Edge. Stone Edge is the usual coverage move, meant to catch stuff like
Volcanion,
Dragonite, and
Salamence on the switch. Shell Side Arm is currently the signature move of
Galarian Slowbro, and it uses whichever attacking stat will do more damage. This is a perfect pairing with Download, and helps Clawitzer against Fairy types which resist its pulse moves, such as
Azumarill,
Florges, and 
Sylveon.
One problem I always had with Clawitzer was that it was too slow to outspeed and KO most opponents, yet it did not have enough bulk or recovery to take many hits. Mega Evolution naturally alleviates this, as it now has enough Speed to outspeed upto
Rotom Wash, and plenty of bulk to go alongside that. As a matter of fact, this allows unusual sets like Swords Dance or Dragon Dance, which can double as a revenge killer through Aqua Jet. So yeah, on the whole, Download gives Clawitzer the ability to make use of all its tools and shine.
Sandy Cloak Wormadam, 
Drifblim, and 
Heliolisk! 
