@Sheenica No worries, it would all depend on your personal circumstances and not every one who studied in Au is eligible for the 485 visa. To check your eligibility, you can see this link:
https://www.border.gov.au/Trav/Visa-1/485-
If you think that you'd be needing some more time to prepare for your PR Application (which most of the time is the case), it would be best to apply for 485, though its not obligatory.
Applying for PR takes time because before you can apply for it, you may need to undergo an assessment for your skills, registration, etc. which may take weeks or even months to comply depending on your chosen occupation. You would also need to wait to be invited which again can take weeks or months before you can actually lodge a PR Visa application. Another good thing with 485, is it gives you time to look for a job and work full time.
Without a valid visa, how can you stay here in Au and process all those. You can always lodge your PR anytime even if you have the 485 anyway. 🙂
You're actually lucky if you're eligible for 485 because that was our dilemma before. We needed a visa to transition our student visa to PR.