iNaturalist sightings are fed into gbif.org and obis.org, the two major databases of biological sightings that collate everything. From my experience in my niche, iNaturalist sightings generally agree with the 'professional' sightings in the same database (various scientific expeditions' sightings data). It's definitely valuable data!

Of course, it has some interesting biases, for starters, small species and fast species are generally not submitted because it's hard to take good photos. But scientific expeditions also have their own biases; if they use underwater nets, for example, they'll also select for larger animals.

Some data is far better than no data!

See the sister comment by a_bonobo for a better insight than I have, since I'm primarily focused on biomedicine rather than ecology. My guess is that ecology (which I was peripherally involved in grad school) is one of the areas where amateurs could have the biggest impact, largely because the field is so woefully underfunded and fieldwork requires more dedication and time than money, other than living expenses! Collaborating on an existing academic project might be a great way to get started, both for training and learning about the open questions in the field.