Catching up on some blogs, and I saw this 12c gem by Syed Jaffar Hussain.
Sequences are great. But sometimes they're just a little more unique than you actually need. Sort of like GUIDs. Start churning through them and they're quickly nine or ten digits. There's nothing wrong with long keys from a technical perspective as the extra byte or two of storage is rarely significant. But they can be greedy from a screen layout point of view (especially in tabular forms). And there's a greater chance of getting digits mixed up in email or conversations about them.
If you are doing a nightly or weekly load, it can be nice to load up your half-a-million rows with a 'batch id' and a 'sequence in batch'. Session based sequences are a nice concept that give you scope for keeping those values in a smaller range.
Another trick for reducing the size of IDs is to covert them into HEX. You'll probably only save one character there though, but the mix of alphabetic and numeric characters. TO_CHAR and TO_NUMBER are both happy to use 'XXXXXXXX' format masks to convert between decimal and hex.