Am I doing something wrong with snow foam

As with the advice above look to a lance with adjustable spray and mix rate. Also, and I only ask to be sure, are you diluting the snow foam in the bottle. It shouldn't be "neat". Most foams will use either 100 or 200ml per 500ml of water.

I have a few types and one uses 200ml of foam to 500ml of water and the other is 100ml foam to 500ml of water. The one that uses 200ml of foam comes in 2.5 litre bottles, while the 100ml bottle comes in 1 litre bottles.

This is where the adjustable foam lance comes in handy. You can adjust the rate at which the mixture is "let out" using the dial on top of the lance while the front nozzle adjusts the spray patterns from narrow (which will empty the bottle quickly) to wide spray which can cover the entire car with less than 500ml of water/foam.

Yeah, don't. It'll use far too much, won't give a proper foam, and it'll cost a bloody fortune as you'll run through the average 1 litre bottle with two washes, or fewer.

Mix it 100ml to 500ml of water. If you find that too runny, then adjust the mixture but always mix water with the foam. My 2.5 litre bottle (Autoglym) gets me about25 washes as I only mix it 100ml to 400ml of water. With my lance set up with a fine/wide spray and medium/heavy mix (setting on the lance) I get a nice foamy spray that last about 10 to 15 minutes (weather dependent) and covers the entire car.

I only use the foam if there is a heavy layer of muck/dirt and its a dry day so as to properly soak the dirt making it easier to remove with the initial power hosing. If its just a normal wash with no great amount of dirt then I use a different foam that is actually for hand washing, rather than pre-wash. In that case I simply power hose the car with no foam.

In short its not necessary to foam each and every time.

To be fair, thats not a strong argument against using it. You have done 99% of the "work" by bringing out your power washer, and setting it up. The snow foam lance is an attachment so having to set that up leaves very little in terms of extra work to use the snow foam lance. As for the foam on the shoes, well its very little difference to wet shoes from the water of just washing it compared to the foam, and both can be sorted with proper footwear.

As for its benefits, its a paint/finish protection step. As I said above I don't always use the foam to soak the car unless the fouling is medium to heavy, but the foam stays on the car longer which gives a better soaking and better loosening of the dirt on the car. Plus when it comes to rinsing it off the foam will protect the bodywork from bits of dirt/grit which is removed at "high speed" using the power washer. IOW the foam acts as a barrier encapsulating the dirt and preventing the possible sandblasting effect of dirt during power hosing.

Its up to each whether they see the benefit of it and I'm not trying to change anyone's mind nor convince anyone to use it if they don't see any benefit. For me its a simple case of a simple extra step, low cost, and some peace of mind I'm doing right. Maybe its snake oil, but if so then I guess I'm a willing fool.