It is usually said that if a directory has 777 permission , the world can write to it hence it is not supported.
Suppose I have domain like abc.com/DIR
DIR is the directory and has 777 permission, how anyonce can write to that directory without FTP Access ?
The major problem with adding write access for others is that any process running on the same machine can write into this directory.
By default apache, nginx, ... do not allow to upload files to a server and lets assume your server is configured that way, then you might think a write right for others might not be a problem, but that's not true.
Lets asume you have an application that itself does not execute or evaluate any scripts but somehow writes files based on some external input, then someone might find a way to control what this application writes on to the hard disk. This won't be a problem if the app will run with its own user and cannot write anywhere.
But now there is a folder with write rights for others so this app could write there, e.g. placing a php file in this directory. After the file is place there it can be evaluated/executed using apache.
A rule of thumb is always to run process with the least right they need, and the same is for the rights of the directories. If a web server only should reads the files then set owner of those files to a different user then the web-server, and only make them readable using the group rights.