What is a good point to put on your snow shoes?
If the snow is up to my ankles, I feel almost no extra resistance when hiking. If it's about 20 cm, walking is more exhausting, however, snow shoes also cause some extra strain. If I sink up to my knees, it's quite obvious, snow shoes are a great help.
From which point is it more effective to go with snow shoes than without them? Is there a rule of the thumb, or is it fully individual?
I used to run outdoor orienteering constests in winter at a boarding school. The kids had to bring snowshoes, but whether they wore them was a team decision. At 2-3 inches no one wore snowshoes. At 8 inches almost everyone did.
The big difference for me was the ability of snowshoes to bridge things you couldn't see under the snow. Rocks and pocket gopher mounds in pasture and small logs in bush; hummock bogs, were the main things.
If on established trail, the boundary happened when you had to consciously lift your foot up to place it, rather than the 'just enough to clear the dirt' that is a normal walking pace.
Another determinate is the size of your group. Snowshoes are a win at shallower depths with a larger group. The first two people create a trail. Everyone behind them just walks on snowshoes.
On crusted snow, and frozen creeks, snowshoes give traction. And they help on elevator snow. (The snow that has a crust almost strong enough to hold your weight)
You can also get a situation where the crust will 'almost' support you on snowshoes. This often results in your tips getting caught under the crust. Sharply upturned wooden snowshoes made from separate side rails (Ojibway style) have less problem this way,
Note that on broken trail, snowshoes can be faster than walking. In our winter program the winning time on the 47 mile senior race was 12 hours and change, with most teams coming in in under 15 hours. (Boys, grades 10-12, teams of 5-7 that had to move as a unit, 1 hour stop for lunch, 30 minute stop for supper.)
It depends on a lot of factors, and I don't think there are any general rules. As you've noted, snowshoes are mainly for deep, soft snow that you'd otherwise posthole in. If the conditions are icy, you want crampons. If it's 50% dirt and 50% snow, you want boots or microspikes. Some snowshoes have heel lifts and built-in crampons, which makes them more usable on steep terrain or somewhat icy conditions. If your snowshoes don't have those features, then you may find they're useless on steep terrain. If you're on a trail that's too narrow, with steep slopes on either side, you may find that you can't manage it with snowshoes because there's not enough room. If there's a nice boot track up a hill, you may want to walk in that and not use the snowshoes. For cruising down a steep snow field, you might find it just as easy to plunge step in boots as to use snowshoes.