Let me make an analogy. Your house is like your OS, your house can be very secure, or not at all, it can be very customisable or completely pre-made, it can be completely a black box where you never know how it's wired or where are the pipes and for some reason the kitchen lights turn off the bathroom ones and it's illegal to open the walls, or it can be an open plan where everyone can know how it's wired and where the pipes go.

The internet is the post network, you can send letters to anyone in the world. Your local post office branch is your ISP, you send letters to them, and they redirect it to whoever you addressed them to.

HTTP is like writing letters in plain English and address them to the person you want. If your local post office wanted they can open the letter, read what you wrote, and know what you said and to who.

HTTPS is like writing a letter in a code/encryption only you and the person you're sending the letter to know. Your local post office knows who you're sending it to, but can't know what you're saying.

A VPN is like a friend who go you can send letters to asking him to send a letter to someone else. Now if the post office sees your letters they can't know to who they're actually addressed nor what they say.

Torrenting is like posting things to a bulletin board, except this is done through the post. The content of the bulletin board are public, so you can't hide the information that you want to watch X movie, if you send the letter through the post they know that you're sending it to the bulletin board, and even if it was encrypted they can read it, what's more when someone sees the board and replies to you the post know what he's replying to, so they know you're getting the movie you asked for.

On this analogy, does it really matter how secure your house is? Not really. The only way you're safe is by instead of posting it on the board yourself asking a friend (VPN) to post it for you and receive the responses and send them to you, and between you and your friend you have a code (HTTPS) so that the post office can't read what you are talking about.

There are reasons why you would want to have a VPN outside torrenting, if you use public internet, e.g. coffee shops or shopping malls, you should be very careful, because on those networks someone can stand between you and who you're trying to contact and pretend to be them and you would be giving your password and bank details to that person thinking you're talking to the bank. This is called a man-in-the-middle attack, but if you're sending those details to your friend for him to send to the bank you know your friend's address so that's harder to take, also your connection to him is encrypted, and your bank details are encrypted so that not even your friend can read them, so it's extra secure.

Also a VPN is not that expensive, the one I use costs €49 a year, if you go monthly it's €7, for the security they offer it's a no brainer.