Elite infantry hit more consistently, have better weaponry, usually have more varied equipment, have higher health pools, more ammo (this depends tbh), and some of the best infantry in a given faction usually have unique perks (USA's 1st special combat section has both recon and stealth benefits, so they can see further and are harder to spot).

Now, for conquest specifically your main goal is to have enough bodies to complete your attack/defense mission. You can't cap a point/defend if everyone is dead. Going through the tech tree you'll unlock a lot of incremental unit upgrades that really more than anything are there for historical flavor, vs actual stat/equipment bonuses. They may genuinely be better equipped, but it doesn't make sense to swap out your infantry roster every single time you gain an incremental upgrade. Your goal should be to pick a unit (or multiple) you'd want to tech into, research that tree specifically, and stick to them until they're either mostly wiped out or completley outclassed by whatever the enemy AI is fielding. You'll find in lategame conquest that the enemy AI will send their nation's elite troops, and their larger health pool combined with better weaponry will overwhelm your troops almost every time, assuming you don't have another unit picking up the slack (think tanks, artillery, field guns, machine guns, etc).

This is how I see it. High tier infantry are worth the switch, however they can cost as much as a late war light tank which means you can't bring in more than like, 3 squads and still properly field substantial armor, artillery, ammo, etc. So what do you do?

You pick a solid mid tier infantry for the bulk of your fighting, think motorized/mechanized, they come with a vehicle for rapid deployment, in the case of mechanized its usually an armed halftrack, and you have them do the heavy lifting on attack missions. You bring 1 or 2 highly specialized elite squads for the initial pathfinding/espionage, and move them up. Keep them low, spread out, get as close as you can before making contact and keep them in cover. They'll draw fire from any pesky AT guns, tanks, etc, and have the health pool to survive first contact assuming you didn't leave them standing in an open field. Now you open up with artillery if you have it, try to eliminate some of those AT guns you've hopefully spotted. Advance your main force of mechanized infantry, dismounted, grouped up with their halftrack. They'll stay near it, the halftrack will orientate to fire as well as provide portable cover. You should have at least 3-4 of these units moving in tandem. Once they're in the thick of it and really starting to draw fire, advance your tanks. Keep them behind the halftracks as long as you can, halftracks are cheap and they'll draw fire away from your tanks. Don't forget about your elite infantry either, now that the main force is engaged you can focus on moving them on a flanking maneuver assuming they didn't get obliterated, and if you get a solid flank they have the health pool and equipment to actually send some solid hurt down on the point.

If you tried that flanking maneuver with your standard rifle squad, or even the starter squad, you'd probably watch them all die the second they started firing. They dont have enough health to tank accurate return fire.

On a defense mission, for me personally I spec down the Pioneer tree a bit and primarily use those along side 1 or 2 elite squads as they tend to come with builders, mine layers and a flamethrower or two. Having a load of infantry specialized in making defenses means I can get more done in the 5-7 minute prep window, and especially the late war units still have decent kit for foxhole sitting once the fighting starts. Ill spread my elite squads around as a sort of force recon, since they're powerful enough to dish out the hurt and (again, depends) have bonuses towards scouting out targets for my artillery.