In my research area (software engineering), points why people submit to workshops include the following:
Lower barrier for acceptance: Some works are inherently hard to get accepted at top-notch venues, for example, because they are hard to evaluate properly. A paper that was rejected multiple times at higher-level venues might eventually end up at a workshop, so that the authors can still make a (albeit less valuable) publication out of it.
Publication strategies: Some researchers actively try to build a prolific track record of many small (including workshop-level) publications, rather than a few big ones. While it's debatable whether such strategy is best for advancing science, and even for convincing a selection committee, there are certainly some associated benefits, for example, plenty of opportunity for self-citation. Strategic publications may also be used to show an association between two authors (to convince a grant committee that they can work together well enough to write a paper), or to establish an association between the authors and the topic (to show a grant committee that they have some early work in the field).
Getting feedback: Sometimes, people have a nice idea, but are stuck in an early stage of implementing this idea. Workshops are a perfect opportunity for gathering feedback, since they (i) are designed to enable a focused discussion (small audience, specific focus), (ii) usually have a faster review cycle than conferences, and (iii) sometimes allow for short discussion or problem statement papers.
Opportunity to visit the host conference: Workshops are usually embedded into the ecosystem of a larger host conference or conference series. When a researcher wants to visit the host conference, but didn't get a paper accepted there, a workshop paper can be their entry ticket. (It's usually easier to get a conference trip funded if you have a paper at the conference or one of its workshops.)
Warm-up publishing: When joining a new field, a group of authors might want to get accustomed to the associated community to understand their culture, values and standards. Workshops are particularly suitable for this purpose due to high acceptance rates, interactive formats and the opportunity to visit the host conference.