The Passage of time is of huge importance in your RPG games. It’s also often overlooked or relegated to a footnote in my RPG games. Yet unless you are running a one-off game it is of vital importance in an RPG.  If there is no passage of time in your games then there can never be any sense of urgency in your games. Also if you have no passage of time in your games and you then Interject time into your game for the purpose of one or two sessions it then feels like little more than a plot device.
There are many downsides to not having the passage of time in your games. Here are just a few off the top of my head.

• No one ages, No one dies of natural causes unless the GM its plot worthy
• No NPC family ever has children, no children ever grow up to adulthood & NPC friends are forever frozen in time from the moment the player meets them and never improve age or die.
•  Villages never grow into bustling cities or fade away
• The Ruling Class never loses power – unless directly due to player activity or scripted events.
• Quest lines just sit in idle until the players become involved if ever.
• NPC bad Guys stay at one level of ability. Don’t worry that NPC that nearly wiped your party. You can come back in 6 or 7 levels and defeat him in  your sleep.

When time passes in your game you add an entirely  new level of depth to your world. Decisions the players make now can have much wider implications.

• By making choices of who in the world to help or not. These actions could have massive implications. Say the PC’s chose to delve into that dungeon instead of seeking out the nearby bandit camp.They might return to a village in flames!
• Maybe since the player group would not deal with the bandits, another adventuring group takes care of the bandits. Leaving the players on the outs with many of the locals.
• NPC bad guys in these style settings not only grow in power but might go on to raise entire armies to continue to hinder the players. Learning from his mistakes if given enough time he will only become more powerful. 
• Days weeks months and years now pass. With that Towns, Citys, Nations or even gods may rise or fall in  your story.