It's possible for PCs to start a campaign at 1st level, embark upon a modest adventure, and find themselves ascending to unfathomable levels of power and renown in just a few months of game time. Although this pace might work for some campaigns, the speed at which the PCs grow in strength and global relevance may be narratively jarring for others. Downtime allow the GM to organically build pauses into the PCs epic journey and moderate the passage of time. It also provide opportunities for PCs to engage in activities that take longer than the
time scales of adventuring can accommodate.
You may initiate a period of downtime organically between trips to dungeons and battles against ancient evils. You might need time to rest, recuperate, and prepare for your next adventure. You can also use this time to perform other tasks, such as crafting arms and armor, performing research, training, networking, or spending your hard-earned gold.
The GM may also initiate a period of downtime between adventures. The GM might simply declare that a certain amount of time has passed and allow you to describe in general terms what activities your was doing during that time. At other times, the GM might want to keep track of just how much time is passing as events beyond your perception stay in motion.
In either case, downtime could be something that occurs with as little or as much fanfare and description as you and the GM are interested in engaging in.