I wonder now and then, why this question is still open
Usually, MO's members tend to be very strict about what is and what isn't a good question.
But why is this a good question?
D Clausen has clarified the controversial nature of the quote, regarding the content of the quote I really wonder if the OP has at least read RetS, where Grothendieck explains in detail why he thinks topos is a deep notion, and in fact a transformation of the very nature of space. It's ok If the Op doesn't agree with G's digression, but the question is not even a historical one.
Usually, we see questions of people that have not encountered an object in their research and they think that the object is unmotivated, or suggest a lack of imagination of the mathematician that creates it.
Topos theory now has a community, with international congresses and a wealth of publications and applications.
Why this question is research-level mathematics?