I can't really answer this question in general, but I can speak for myself: while it's not something a lot of PhD students do in Belgium (it seems to be more common in the US actually), I felt like it would be a great opportunity to experience what working in industry is like. When I finish my PhD, I'll have to decide whether I want to stay in academia or move to industry, and this internship is hopefully going to help me make that decision.
You can get research internships at large companies. If your PhD grant doesn't cover summer at full-time rates, SV companies pay on the order of $5k-$8k per MONTH for interns. Actually, even if your PhD grant covers your summer expenses, it probably won't match the internship salary.