why the heck would the Imperium use siege armys? after world war 1 3 invention made trench warfare impossable, first: the tank and we know the Imperium has oodles of tanks, second: improments air craft percisely speaking bombers and the Imperium has good Bommbers too, Third: improvments to artilary leathlity and improved artilary tatics, both the Imperium has.
in the Cold war Thermobaric weapons were invented, which are to quto Wikipedia , "thermobaric weapon, which includes the type known as a "fuel-air bomb", is an explosive weapon that produces a blast wave of a significantly longer duration than those produced by condensed explosives." the side effect of the fire ball is a massive vaccum created by the the fire bal sucking in all the air, you sufficate and in extream case can have the lungs ripped out of you,
this works well on people dug into tunnels or caves, or bunkers. and lests not forget bunker buster bombs which can rip though 20 feet of croncrete or world war 2 earth quake bombs which upon impact with earth dig in, the expode makeing a earth quake that dose the damage. and if it can destroy a rail road tunnel by going 60 feet though
the hill it will destroy a trench.
whith these advancemts why the heck do you need seige regimentes? heck with as many armor and mecinized reiments as seige regiments were deploy I could have ended the war for Vraks (imperal armor 5,6,7) in 3 years.