I am partly generalizing current replies but added some new ones. I am not sure yet which unit correspond to my needs, perhaps just trying the cheapest one like this (2.75USD, shipped). I can imagine all of my usage cases below would be satisfied with an odometer.
I bought my bike as second-hand and it made a good impression when the seller gave all stats about the bike usage like 10k driven, it is 2 years old but been in heavy use. I calculated the price with
25% demortization rate p.a. (pessimistily to 3 years) and added a risk factor that depended on the driven distance. So it was easy for me to pay a fair price. Please, do not interpret the last thing wrong. I bought the bike from auction-site and since I knew the
lawful price for it, I made considerably higher bid than the next highest bid (not wanting to waste time) so I think a good example where quantifying bike usage is good for both parties. And am I happy? Sure I am, superb bike and I do like fixing some trivial bugs in it like fenders and pedals.
Seen suggestions such as lubricate every 300miles (about every 500km).
You don't always know places, exactly, just something like 4.2 km along that street and then left. If you know exactly your driven distance and don't want to look at the map all the time, I think odometer can make the journey much more pleasant, killing unnecessary zig-zagging.