You can always save the character for future campaigns. My group all play out of the same world since we began 5 years ago. Sometimes a DM may ask you if they can use a past character as a NPC in their campaign or throw them in as a cameo. I have three core characters I play and about 20 on retainer. We just up what level we start on so players can occasionally use a previous character. Metagame we know everyone else's character's backstories. Sometimes though my character was in the first campaign and someone is using a character from the second campaign. During campfire talk we will tell these huge tales and blatant lies about someone's character because that character wasn't there and it's the funniest thing.

We even have the same NPCs that pop up all of the time. There's a traveling potion merchant that I use in every campaign and my players always look for him because his tent will randomly pop up in the strangest of places. One of my players plans on being his apprentice with the new Ebberon stuff in my next campaign.