Well lets say infantry must take a city. Why bother going from street to street whit infantry exposing to fire when could just blast the area whit a rocket?
Kinda depends on why you want to take control of that city... If you are Georgia and you want to take cities and towns in South Ossetia and Abkhazia then it kinda looks bad if you kill all the civilians you are pretending you want to liberate.
If it is a foriegn city with hostile natives who want to kill your people... then do what the Germans did to Stalingrad... level it with air power and then send in troops to occupy the rubble... except even reducing a city to rubble doesn't kill everyone in that city and then the air raid is over all that rubble offers excellent cover and concealment for defenders and limits mobility of heavy vehicles for the attackers.
Or you are on flat land searching for enemys.
Even flat land has trees and things to hide behind... you wont have enough ammo to level every tree....
You get the information you wait until they reach your target area then you hit them. Ofcourse tanks will not come alone but neither you.
What makes you think when you can hit them they can't hit you?
More importantly they are heavily armoured and protected so just any old missile wont defeat them easily. You on the other hand are in trucks... easy kills.
Question: how you defend against projectiles shooted from far away like 40km by enemy artilery?
Your average artillery fired projectile starts slowing down as it leaves the muzzle of the weapon that fires it... modern APS should be effective and with guided rounds things like Shtora should also be effective.
More importantly local artillery should detect the incoming rounds... determine the source of fire and have retaliatory fire on the way before the enemy shells land.
For large shells... ie 152mm round TOR and Pantsir should be effective.
can change of center of gravity of this kind of projectile or a missile be a kind of guidance?
No. A change in cg is not related to the targets position and there will be no correlation between those two things... in simple terms the amount of fuel left in the fuel tank of your car cannot be used as a guidance system to determine how close or how far you are from your destination except to tell you how far you have travelled... if you take a wrong turn or you car uses a lot of fuel you might find you have to refill quite a bit for a long trip... but most guided rounds might use a base bleed or rocket assistance to extend range... neither of which will tell you very much except that rocket assistance burns out in less than ten seconds so you know you will probably be about 8-10km from the launch gun when it burns out.... but that would be the same for a target that is 40km away or 60km away.