From Lostpedia

• Daniel needs the journal because of his apparent memory problems.
• Daniel received the journal from a future and more experienced version of himself.
• Daniel and the rest of those that stayed on the island stopped skipping through time when Jack and crew returned to the island via flight 316. Daniel got himself recruited as a workman (as seen in the opening scene of season 5). According to how Lost treats causality, sometime after he sees the Orchid station, Daniel would have hidden the journal right where he found it in the 'future' (past to him), then he would have to begin writing the journal anew so that it could be written in the first place.--.
• Daniel's journal will eventually become known as the Black Rock ledger
• The Journal is in a time loop. Daniel adds new things to it every loop, and his memory problems exist to prevent him from knowing the future. Eventually (possibly what we are seeing in the show), the information in the journal will end the time loop.