This is for when you can't "just sell the electricity to the grid", either because it is congested or because you have more electricity at the moment than there is consumer demand available. An example off the top of my mind is in the North of the Netherlands, where the power cables from several major wind parks will come ashore. During periods of high production and limited demand, like a storm during the weekend, there will be more production than they can shift with the infra available. Shell is building a multi-GW electrolyzer plant to absorb the excess energy.

There are also some industrial applications like steel smelting where some chemical reducing agent is required, which would be difficult to convert to electricity-only operation. Fertilizer production is also pretty hydrogen-heavy.