Dandelion seeds are a local specialty to Mondstadt that can be farmed from the Dandelion Plants in the region. Local specialty means you can only find Dandelion seeds in the Mondstadt region. Like, Jueyun chili exists only in the Liyue region; the same is true for Dandelion seeds. You have to apply anemo element on the Dandelion plants to collect them. If you have characters like Jean or Eula, you need Dandelion seeds for their ascension. I'd recommend you to go through our guide on Dandelion seeds on the site to learn more about them and start collecting them if you have any characters that use them in their ascension. I won't recommend you stack them up in your inventory, rather, have a few of them as you need to give them to some NPCs in certain quests. Sometimes, daily commissions require you to interact with NPCs in the daily quests. You have to submit certain materials to them. Dandelion seeds can be used to submit to some NPCs, and I highly recommend you keep a few of them in your inventory.