Hubby has been prescribed these high energy foods/drinks. The drinks are OK but a little strange in texture. He prefers the desserts which are much more palatable. They are not a meal replacement but a meal supplement so, as you say, they help to increase weight although he has them for weight maintenance. They are also fortified with vitamins & minerals etc. Hubby sometimes has one for breakfast if he doesn’t feel like eating anything else.
They are available in France if you want to look them up. Here they are called Fortimel.
NestleHealthScience produce similar products as well. Their desserts are particularly good.