If you want to break into copywriting, becoming a junior copywriter is one good way to get exposure and knowledge. This is your internship into what can be a lucrative job. Trying to break into copywriting on your own, is much more difficult without some kind of portfolio of work.
If you can obtain a position as a junior copywriter, you are one step ahead of the rest that are trying to make it on their own. You can learn from industry professionals. What a great way to learn the ins and outs, by actually seeing in in front of you and observing someone that does it daily. Once you have some experience under your belt, you can move up or strike out on your own with a portfolio of work to show prospective clients on a competitive company if you wish.
If you want a business of your own, copywriting is a good one. There are many companies that need good copywriting for their brochures, pamphlets and other advertising materials. There is copywriting work on behalf of non-profit entities, like animal rescues and charities that are always looking for new avenues of fund raising. Good copywriting is one way.
Copywriting is and always will be a niche position for good writers of copy for businesses, a local chamber of commerce, hotels, travel agencies and so many more. One could make a decent living by being a copywriter. Starting as a junior copywriter is certainly a good way to garner experience and get your questions answered; instead of having to learn the answers the hard way.
Being a junior copywriter for a large company would certainly allow you to learn and ask question as you build a portfolio of work. You would also be brainstorming with experienced copywriters and see how the targeted wording works for a business or it doesn't. Not every campaign of copywriting will work every time. That just isn't logical. They will get people in, but will they spend money? The gurus say it will if you use the right approach. You have to have something they want to spend on or donate to for the words to work.
If you can get a position as a junior copywriter because you would like to, go for it and learn your trade well.