Just to add to what Scroggs says, if you arrive off the airways at an airfield in the UK that has STARs published, but have filed direct, in NATS' wonderful computer system will put you on a STAR even though you haven't filed on one, therefore controllers are likely to expect you to follow the STAR unless otherwise vectored.
The significance of STARs in radio fail procedures hasn't yet been mentioned. If you go radio fail in the cruise, you'll be expected to follow the vertical and lateral profile of the STAR and (in the UK at least) to hold at the end of it until your ETA or last acknowledged Expected Approach Time (if given one by ATC), thence commence a procedural approach for the duty runway at your destination.