Textra is noticeably experiencing worse speed and fluidity when text in conversations get too long (you many messages in one convo). For example, there is a delay between pressing send for a message and the message appearing in the convo for longer conversations. Therefore, if all messages can be stored on the SD card instead of in the limited internal storage, Textra can just "read" the messages and won't be as bogged down, increasing both app speed AND functionality (larger storage = more room to save important conversations).