Some exchanges and wallets use a shared wallet and database. A memo ID is used to determine which assets belong to who.
Important note for Stronghold customers:
We require customers to enter a Memo ID or Tag, along with your deposit address when depositing XRP, XLM and other Stellar based assets. Omitting a Memo ID can result in a loss of funds, so please note this when making future deposits.
When withdrawing Lumens, the option to add a memo ID will also display.