Students usually have no difficulty asserting an opinion. Supporting that opinion is an entirely different matter.
Supporting an opinion means sticking to one idea. Beginning writers are not very good at that. Their writing is usually stream of consciousness with misspellings rather than logical reasoning with supporting evidence.
Having a good working thesis statement, however, helps student writers determine what to put in their essays and what to leave out. Savvy students (who typically are students taught by a savvy writing teacher) select a working thesis to use in thinking about their writing project.
If the thesis statement turns out to be workable, they have the biggest chunk of their writing planned. If it turns out to be unworkable, they haven’t wasted hours composing and their thinking can usually be turned into a workable plan without a great deal more effort.