A wider lens (f/2.8 @ 17 mm vs f/3.5 at 18mm, f/4 vs f/5.6 at the tele end) can give you possibilities which you wouldn't have otherwise, in terms of depth of field and/or of bokeh (greater presence of roundly-shaped out-of-focus highlights). This can help your inspiration but it depends on the kind of shots that you are aiming for. Besides, you will be able to use the lens in situation with reduced light (like indoor). This applies more to the wide end of the lens (17mm) where distortion will be more pronounced: so keep it in mind if you plan to use this lens for indoor portraits without flash or other lightning gear.
The greater focal length, in the same way, can be used for tighter framing, which can be useful e.g. for portraits or, as this blog post shows, for a different take on landscape (but 70mm is probably a bit too short for that, in my experience).
So the Sigma lens obviously surely offers some capabilities that your current kit lens doesn't have or offer in the same amount. If it is worth a given amount of euros depends on your commitment to the hobby and to your satisfaction with the pics that you are able to take (or dissatisfaction with those that you are unable to).