This is always a great question for any Tarot enthusiast to be asking.
My own take on it says first of all that no, you don't have to own a copy of the RWS. If you want to advance in your Tarot practice, you almost certainly have to own *a* deck -- I can't imagine just studying scans of cards online, and using deck apps on your phone as your only interactions with the cards and their images.
But after resolving to buy an actual, physical deck...more important than "Which deck is the most popular?" or "Which deck came first?" is the question of "Which deck will keep me the most engaged?" If you buy a deck because everyone else says you should buy it, or because the one you pick seems like it's "scholarly" or whatever, you might end up feeling bored by it, and obligated to use it, like a kid resentfully slogging through homework. You won't enjoy it, and your enthusiasm for Tarot may get rather chilled...and how does that help anyone?
Personally, I think you (the editorial you -- anyone!) should get the deck that most excites you, and that will have you not just working with Tarot, but devouring it.
But then here's the second part... As mentioned up-thread, most decks are based on the RWS, the Thoth, or the TdM. Figure out which one serves as the template for the deck you got that excites you, and as you get familiar with your deck, start holding it up against the blueprint. That is, if your deck is RWS-based, study it and play with it and read with it...and also begin to slowly compare it with the RWS (you can do this entirely online if you want -- the RWS cards are scanned all over the place, and discussed just as widely). How are the two decks' versions of the same card similar? How are they different? What's the "generally accepted" meaning for the card that you see most often out on the internet, and how well does each of the two versions (your deck's and the RWS') embody that meaning? What meanings do you see in the card that no one else seems to talk about, and why?
Examining the deck against its source is an incredible tool for learning about your deck, for learning about the original that gave rise to it, and for learning about yourself (as a reader, and as a human).
And as lots of people have also said, working with one of the "Big Three" decks gives you a common language. It allows you to go and reality-check your understandings with other people, it makes tons of free resources available to you, and it can help you to feel connected to a community of sorts if that matters to you. It can also connect you more powerfully to the phenomenon of Tarot in its entirety -- its history, its evolution, its possibilities.
But even just practically speaking, consider that if you ask the population of a message board like this one why the person in the RWS 2 of Swords is blindfolded, versus why the person in the RWS 8 of Swords is also blindfolded, you'll get a flood of answers to consider. But if you ask why, say, there are black and white objects offsetting each other on opposite sides of both the Royal Maze card and the Citadel card in the I Am One Tarot, you're likely to hear only the chirping of distant crickets in reply...