Now Im not 100% certain how 40k space battles take place but I have to assume that it does not hinge on boarding
Even a small number of Space Marines can disable a vessel. Look at the battle of Crythe in Soul Hunter
. A few squads of Night Lords Astartes were able to disable a Grand Cruiser in a matter of minutes by destroying its bridge and secondary enginarium.
In the later orbital battle at Crythe, Abaddon's forces (with the extremely powerful Venegeful Spirit involved) outnumbered the Loyalists forces (whom were mostly made of strike cruisers, too). Their firepower in a traditional sense would have annihilated the Loyalists forces.
The difference was the Black Legion vessels were empty of Astartes. The Black Legion had to retreat or risk destruction by being boarded.
Beyond the usefulness of boarding Ork vessels (say, killing key command ships and watching the Orks fall on themselves to find a new leader), it's pretty clear the Orks themselves would be eager to board Imperial vessels. The Astartes on board the ships would be both a sword *and* shield for the fleet.
In The Emperor's Gift
an Inquisitor ordered Astartes to board enemy cruisers and battleships to "kill them from within".
Also in Legion
we see Space Marines boarding a Expeditionary Fleet flagship and were pretty much unstoppable. They took out the Lord Commander with apparent ease.
And in Battle for the Abyss
the most impressively built Furious Abyss
was taken out by a comparably small number of Legionaires. Keep in mind two Furious Abyss
class vessels ran amok over Armatura, one of the key war-making planets in the 500 Worlds.
Boarding and planetary assaults are what Space Marine vessels were built for.