Objective ways Openstreetmap is better:

• More regularly updated (one street I used to live on in Amsterdam -- a cosmopolis -- has only recently been added to Google Maps, whereas I used Openstreetmap to find my way there when I moved in four years ago).
• People who update it get credited rather than an already disgustingly rich company; you can submit edits to Google Maps but they often don't get implemented, at least not with any alacrity, and they keep the intellectual property rights to the data you submit.
• Better for privacy (probably goes without saying).
• Open-source, for what it's worth.
• The map is colour-coded and actually easily legible rather than every way being a white or light grey, sometimes hair-thin line on a white background.
• Actually useful for people not sitting in cars as it shows pedestrian ways, cycleways, parks in detail, crossings, gates, stiles, etc.
• Useful for non-navigation purposes, in fact it's the map of choice for people gathering map data.
• The directions don't send you the wrong way up one-way streets or along roads you can't ride your vehicle on, among other mistakes.
• Openstreetmap shows public amenities like bins, water fountains, benches, etc. I've used it to help my dad who has Parkinson's get a quick bit of rest while visiting cities, for example.
• Google Maps is admittedly quicker for looking for branches of big companies, but you can do that without a map, and Google Maps is chock full of random businesses registered at people's homes and searches can be obfuscated because richer companies pay to come higher in the search results.
• Google Maps has public transport info, but the info is so often wrong that I would seriously advise against using it.
  • I used to work at a train station and people would come up asking why the train shown on Google Maps wasn't showing on the departure board;
  • I've seen people miss the last train of the night because Google Maps said it was leaving later than it actually was;
  • it often doesn't show the quickest or easiest route and you can't refine the search the way you can on public transport apps,
  • etc. etc. I'd say the info on Google Maps is so bad that it makes Openstreetmap better because it doesn't tempt you with the false promise.
• The other features Google Maps is garnished with aren't really needed if you can read a map or if you just need a map, like:
  • street view (nice to have and has made Geoguessr possible (now pay-to-play in part thanks to Google's closed-source APIs), but itself updated by volunteers who have to resort to things like holding signs with their company names to get credit and only updated every few years or so on average),
  • opening times (whether it's correct information aside, you can just look that up otherwise and get it from the horse's mouth), or
  • searching for something like cafés within a given radius (when's the last time you went to eat out and thought, "any café will do, but I'm slightly pickier than to warrant just walking further up the road to find somewhere, and I can't be arsed just looking at the local area on the map and picking out cafés"? And are those cafés even open when Google Maps says they're open? Not necessarily!).
• I'm sure something else I can't think of at the moment. You can see I've been asked this a fair few times.