I use Merlin as well. It's very accurate at identifying birds by sound but by pictures it's very hit and miss.
I use the BirdTrack app to record my bird sightings. The data is used by and helps the BTO (British Trust of Ornithology). As someone who used to play Pokemon GO it gives me a similar kind of enjoyment.
Merlin also allows you to record bird sightings but that data goes to the Cornell Lab of Ornithology in America instead. I tend to record my sightings in both, but only when I'm independently confident of my identification (i.e. not just plugging in what Merlin is telling me I'm hearing).
I think the RHS (Royal Horticultural Society) has some plant ID apps which I'm tempted to try out since I want to learn more about native plants/trees for my future gardening endeavours.