Because your internet traffic moves to and from your devices (computer, phone, tv, tablet) through your ISP, they can see where you go online. An ISP can see what sites you visit, how long you’re on them, your location and information about your devices. An ISP may not know the specifics of what you did on those sites (like what you bought, searched for or read) thanks to encryption, but they could make inferences about you based on the sites that you visited.  That personal data can be used to create detailed profiles about you. Why would ISPs do that? In short: this data is valuable.
Oherwydd bod eich traffig rhyngrwyd yn symud yn ôl ac ymlaen o'ch dyfeisiau (cyfrifiadur, ffôn, teledu, tabled) trwy'ch ISP, maen nhw'n gallu gweld ble rydych chi'n mynd ar-lein. Gall ISP weld pa wefannau rydych chi'n ymweld â nhw, pa mor hir rydych chi arnyn nhw, eich lleoliad a gwybodaeth am eich dyfeisiau. Efallai na fydd eich ISP yn gwybod manylion yr hyn a wnaethoch ar y gwefannau hynny (fel yr hyn y gwnaethoch ei brynu, chwilio amdano neu ei ddarllen) diolch i amgryptio, ond gallan nhw ddyfalu ar sail y gwefannau yr aethoch chi iddyn nhw. Mae modd defnyddio’r data personol hwnnw i greu proffiliau manwl amdanoch chi. Pam fyddai ISPs yn gwneud hynny? Yn fyr: mae'r data hwn yn werthfawr.