I get that question a lot! And there is 1 or 2 main reasons.
I never used to have 2 lines. Only got it installed when I found out it was thing but it’s been a big help! You see everything in my home is internet based, from TV channels to smart home equipment to multiple game consoles. And at my address the broadband speed has dropped quite a bit from recent years since our cabinet became more full. On a 76mbps download package with both it normally sits around 50 and I started to realise 1 connection at around 50mbps handling all this bandwidth wasn’t good enough. Especially when you’ve got multiple 4K Netflix streams running! So I took out the 2nd line to separate the consoles and PC’s so that bandwidth isn’t further draining our 1st line
Another reason is it’s actually became a really good back up! Before I switched to EE I had TalkTalk on my 1st line. I found every now and then some random fault would appear that only affected their wholesale network on the cabinet. Meaning whilst that was being fixed on I still had a completely working network on my other provider running on Vodafone’s backhaul. It was never necessarily intended to be a backup but damn it was so nice being able to just switch my mesh Wi-Fi gear to the other network and not have no internet throughout the couple hours it was down!