There are loads of different types of traffic. The domain could be getting traffic because:
- it was an old site and there are still incoming links to it and/or content indexed in the search engines
- it's a typo of a popular/well-known site and fat-fingered typing is bringing "lost" visitors
- it's a strong generic and people are typing it in to see what's there
All the above are at least "human" visitors, but then as has been mentioned already there are also the bots. When I did a bit of referral tracking a while back, I found that less than 1% of traffic hitting some domains was actual people, and the rest was search engine spiders, content auditing bots, archive bots, network auditing bots, spambots and other junk traffic.
So take any simplistic visitor count with an absolute mountain of salt!
And remember that if the source of traffic is down to the domain being a typo, the domain could be worthless even if it gets massive numbers of visitors. For example, I bet googel.co.uk gets thousands of typo visits a day, but it's subject to being taken away by Google's lawyers any time (I haven't checked the actual ownership of the domain)
Similarly, if it's an old site then the traffic is going to tail off over time. Meanwhile the domain itself may not be "desirable" because it's too specifically related to a particular defunct business (eg freddysmithglazingservices.co.uk - made up example - might have 200 incoming links from yellow page sites, and consequently some residual traffic, yet nobody other than "freddy smith" is realistically going to want it)