A few reasons. First, I can buy less data than 15Gb, and spend less of course. Secondly, I seldom need anywhere near that much for my uses. Voice service uses around 16Kbps, so about 1Mb per minute. I have several lines that get the 256kbps free data roaming, so that generally works well for texting, email, mapping, navigation, etc. If I am getting free US calling via IMS, I do not need the included voice service of T-Mobile's Data Passes. So I spend $10-$15 and with hotel WiFi mornings & nights, I never need any more than the 1-5Gb I get from Airalo.