Lenin

The high protein vegetable foods are the legumes like beans, peas, lentils, chickpeas. Tofu is a very useful bean sourse that needs no further cooking (all other legumes do.)
As Judy implies, legumes have only a limited amount of the amino acid, methionine and thus to get enough methionine, you need to eat a lot of beans.
Wheat and rice are short of lysine but have less of a methionine shortage than the legumes which easily cover the lysine requirement.
So mixing your plant proteins allows you to eat less.

Let's say you need 40 grams of a perfectly balanced protein (like egg) to maintain nitrogen balance (protein in = protein out.)
If you have only soy at say 60% balanced (just for illustation) you might need 66.7 grams soy protein to get it.
If you eat only wheat (say 40% balanced) you will need 100 grams of wheat protein.
BUT if you eat them together you will need less than 66.7 grams of protein and if you balance perfectly you can eat close to the 40 grams vegetable proteins and thus match the high quality of the egg protein.

BUT remember, you CAN eat the 100 grams of bread protein or the 66.7 grams soy protein and also get all you need...but if you are going to get all your protein from vegetable sources it's wiser to mix your protein sources (legumes + grains + fruits + vegetables.)
If you do this, then you will need less total protein than if your diet was an exclusionary one or two items.

Obviously, the food combining is more improtant in areas of the world with a food shortage. In well-fed (and over-fed) countries one is unlikely to develop a protein shortage no matter WHAT the food choices. Alas, in some parts of the globe getting 1200 calories of rice a day is more than many people can get...then a few beans can mean life or death.