I don't think you have to start writing knowing the ending, but I think its a good idea to start PUBLISHING knowing the ending.
A few reasons:
1) your audience can sense meandering. If your story started with a Big Epic Adventure premise or Gigantic Mystery but you spend pages upon pages spinning your wheels or checking out Sideplot 14, they will lose interest and move on (unless the art is phenomenal, and even then, its a dicey proposition).
2) longer isn't better. It CAN be, but generally speaking, the Lord of the Rings, Harry Potters, Game of Thrones of the world are rare - most truly great stories have a beginning, middle, and end you can read in one book. You can still end a story while allowing room for a sequel.
If you don't plan an ending, you may fall into a trap of not knowing HOW to end. Too many webcomics face this pattern:
-Epic Prologue
-Beginning
-Middle
-Middle
-Random Sideplot
-Middle
-Middle
-Another Random Sideplot
-Middle
-Middle
-Canceled Abruptly/Letdown Ending/Artist Starts New Webcomic/Artist Gets Paying Comic Job Which Prohibits Working On Free Comic/Artist Loses Interest.
I assume most everyone doesn't want to make that comic. The easiest way is to start small, and edit edit edit your story down to the essentials - if you get the main story done, you can always write spinoffs/supporting materials, ancillary stories, back-ups, etc.
I don't mean to sound preachy,and you can do whatever you want to do - but I do feel the best way to set yourself up for success is to finish what you start, and do at least SOME planning ahead.