For example, you get a random string 6dtgw.png for your filename.
I understand a combination of alpha and numeric can tolerate more combinations. But that doesn’t stop a running counter from using alpha numeric.
0001..0008,0009,000a,000A,000b
(Note: I apologize if I made a wrong assumption, as I am assuming image-hosting use a random string as filename. This question will not make sense if my assumption is incorrect.)
I am not sure but i think it has thing to do with decentralized is better, counter is a bottleneck that restricts scalability.
Using a sequence would make it very easy for someone else to scan all uploads, which they probably don’t want. Using a sequence also requires synchronization between different processes (possibly on different servers!) to decide who gets to use the next number.