They ideal for most of them is absolutely that they can be frozen while still alive and unfrozen later. We are nowhere near that technology though so most fallback to the second hope. Yes, that is that when they're unfrozen in the future we can cure whatever it is that killed them. From what I've seen in documentaries, most of the people signing up know it's the world's furthest longshot, but they figure they're dead either way, why not take it? Worst that happens is they stay dead but hopefully science learned something from their body at least, best case is they wake up in the 24½th century and keep on truckin.