1-For your portfolio.
Your practice and efforts will be evident in your portfolio! Wether you are still trying to get into design school or a working professional of many years, I highly recommend always continuing to develop the hand and eye coordination that comes from drawing. Ask a friend to pose wearing different types of fabrics and study how a stiff wool hangs differently than chiffon, all these subtle understandings of fabric will be communicated in your portfolio. When applying for a job, the final decision maker is often the creative director. This person got to that position because of their eye and intelligence, they will be able to see right through someone who takes their passion seriously by the effort put forth in your portfolio sketches.
2- To be a better designer.
Wether you draw figures by hand or with a computer the more times you've looked at a model and how her clothing drapes her body, the better designer you will be. As a designer your job is to decide how exactly the fabric will fall on the body through your design. This is exactly what drawing a live model forces you to focus on.
3-To get a job in a higher end fashion house.
Another often unmentioned benefit of having beautiful sketches in your portfolio is that you will attract the attention of higher end design houses as that is how they tend to work. If you are happy settling for a job in mass market fashion where you are just making slight revisions of technical flats on a computer all day, then don't worry about your figure sketches. But if you aspire to work in a designer company where all the head designers present their ideas in quickly rendered figure sketches like Ralph Lauren or Michael Kors then you must keep at your drawing skills. I was never the best at drawing but after hours and hours of practice I was finally able to come up with a style and workflow that clearly communicated my designs. With practice you'll be able to accomplish this too!
One last point for those who just find drawing very difficult. If you've really given it your best shot and feel that drawing and sketching is not your strength, there is a way to make up for it. Make sure your draping, pattern making, and sewing skills are up to par. Original samples of fabric techniques or knitted swatches in your portfolio can more than make up for illustrations that are not effectively communicating your skills. Everyone is different, as a designer you will not have time to drape every single idea, this is why sketching is important. However there have been many great designers who are well know for very basic flat sketches, their strength is in the pattern making and construction of their design. Claire McCardle is a great example of this. Alexander McQueen is also well know to do his entire design process by draping on a dress form. You will each find your own way, test, explore, and don't get hung up on what doesn't come naturally, with determination, an unstoppable attitude, and some raw talent you will figure it out!