There are two parts to the process: online check in and digital key. Some properties only offer OLCI and some offer both. You can OLCI usually from the day before your stay, it will confirm card details then may or may not let you choose a room. If the property doesn't offer digital key then the process ends here and you have to go to the front desk.
If the property does offer digital key, after OLCI the app will then show that it has requested a key. On the day of your stay the key will usually change status to "Ready to use", which means you can go straight to the room and skip the front desk.
However sometimes the key won't become ready and there will be a message telling you to go to the front desk. The reasons for this may include local law requiring the front desk to see your ID, or hotel policy is to make you present a physical credit card and sign some form, or there is some problem with your reservation. In the USA generally you will get the digital key, but it is kind of pointless in countries like Italy where the front desk must copy your passport.
I've also had problems in some properties where the digital key unlocks my room door, but it doesn't unlock some other doors or lift/elevator which are required to get to the room, which also makes it a bit pointless.