In general, yes, you need a thesis statement for most formal writing assignments. Even in more reflective or casual pieces, a thesis statement can help you clarify what you are saying for yourself and for your reader. Rather than looking for the exception, try writing a thesis statement when you aren't sure it is necessary.
Your reader always needs more guidance through your written work than you suspect. Sign-posts and repetition of a main idea can feel laborious as you are writing, but your readers will often thank you for your guidance. Every paper has a structure, even if it's hidden among the weeds or very disorganized. Making it obvious helps you make a clear and coherent piece. And that's what we want: clarity and coherence.
What does a good thesis statement look like?
A good thesis statement:
- answers your research question
- takes a position that others might challenge
- is specific and focused
- passes the "so what?" test
- indicates how the paper will prove it