No real benefit, but it's essentially free and simple so you needn't feel bad about doing this.
Energy producers (e.g. a wind farm, or a huge converted coal power station which burns "waste" forests) get credits for producing renewable energy which they can sell. A company like bulb buys those credits, and so long as they buy credits which add up to the amount of power their residential customers use they can say they're 100% green.
If most UK residential customers used a green provider there wouldn't be enough credits and that could in theory drive change but in practice most UK residential customers pay their "incumbent" even though that's usually the most expensive and has no benefit whatsoever.
The actual supply of electricity to homes is by a boring but necessary natural monopoly supply company. They're the people who come out if a storm smashes the power lines.
