Not a girl and no it's not as simple as self esteem u/tardigrade81, that's ignorant to say.

Tinder can be a tool for a lot of them and those who use it as one do it with the intention of growing their brand. Gaining a following on Instagram can make one post from you for a company worth hundreds and even thousands of dollars, depending on your influence and the reach you have; especially if you cater to a niche market. A lot of the girls I see advertising their accounts promote health and beauty products, wellness drinks, start up clothing lines, fitness products and many other things they receive for free from the company. The reason Tinder works so well is because the people matching with you are already attracted to you, so some might actually follow your accounts to see more content from you. Even if they don't receive followers or a very little each day, if one of those followers has 15,000 viewers and they like one of your posts, well guess what? Even more advertisement from you to grow your account.

The girls who promote their Snapchat, that can vary. Most are advertising their Patreon, selling their pictures, videos, panties and many other things. Some even do it to grow their following similar to Instagram so that they can sell shoutouts and earn money. Getting 10k, 50k or even 100k daily views can make you an easy hundred dollars for shouting out other people - and people pay/eat that shit up. All of these things they can do can make them more money then they would make at most jobs and that's just how things work now.