physicsae, There's a tag called "nodal analysis" that is attached to your question. You don't write much. But are you looking, specifically, for the application of nodal analysis to this problem? If so, that would justify the use of a ground referenced node. (Other methods don't require it. But nodal does require it because of the rank-nullity theorem.) If you are looking for other methods, such as Norton-Thevenin or mesh, then a single ground reference isn't necessary though for mesh you do start with 0 V somewhere and then work yourself around a loop.)
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