why do i need drop box?

Are you talking about Dropbox.com for easily sharing files between all your Macs, iPhone, iPad devices?

or are you talking about /Users/your_user_name/Public/Drop Box

The commercial product is a nice utility for sharing files between devices, sharing with other users, and as a way to have some stuff backed up in the cloud. The free version gives you 2GB free, but you can buy more.

The ./Public/Drop Box is a place where if you allow for multiple users on your Mac, you can transfer a file from one user to another, where the other user can drop it in that folder, but cannot look inside, and cannot take it out.

If you allow network file sharing, the same can be done, if some access is allowed to that folder.

Otherwise, it is not essential to anything. I know about it, I even helped write a Digital Equipment Corporation OpenVMS Apple FileSharing Protocol compatible file system, where I had to make Drop Box permissions work correctly on the OpenVMS operating system.

But in the 34 years I've been using a Mac, I have never used the Drop Box, except for the testing I did when working for Digital.