Glad you asked buddy
Basically they're completely impartial and their focus is to help you understand your (honest) financial situation and then discuss options available to you... Literally everyone I spoke to whenever I called was excellent, non-judgemental and to the point.
Other than those I spoke to through Gamcare, they were the first people I spoke about my problem with - that split from YOUR world made a huge difference as they only knew as much as I wanted to tell them (about my gambling) but it meant that when talking debt I didn't feel the need to miss anything out so when they asked about what I owed then I was comfortable in telling them EVERYTHING.
So once everything was out in the open and they'd asked me about everything, including what I'd not even considered to be a debt (like my multiple phone contracts which I taken out and then sold the phone) we moved on to a statement of affairs (SOA).
An SOA is basically a look at what you have coming in and what you need each month to pay your priority bills and have a degree of quality of life.
They told me to be realistic and not too strict, so a weekly food shop might be £35 (just me at this point) as opposed to the £15 I had been living off etc. Then after this they came to a single one off payment amount which I had to agree to being affordable.
Affordable for me as it turned out was a debt management plan (DMP) £400 per month which meant that I had £1100 pcm to cover rent, food, fuel bills, clothing and all that stuff (including £100 pcm for social stuff) and then they sent it to each of my creditors.
The best of it was all those payday loans had to stop charging fees and interest, as did my credit cards and other unsecured lenders which IMMEDIATELY lifted so much stress from my shoulders it was unreal - forget having a holiday this was Amazing!
Now it wasn't all easy... I did have to contact my creditors to tell them about the my DMP and a couple of those 'friendly' creditors turned out to be the least pleasant - Very for example were horrible while Wonga were awesome! And for a few months I did get letters about defaults but when I did I'd call Step Change and they'd say "meh, it's no big deal but thanks for letting us know" haha
A few months later and everything settled down, I went to work and got paid at the end of the month at which point a single payment left for Step Change and the rest was mine!
A year(ish) later and I was able to get a contract phone with O2 - life was normalising...
Even when I had a 'blip' and couldn't afford to make a full months payment it was no biggie as I told them and they'd say "meh, no biggie just reduce your payments for a bit" - sometimes I made no payment but it wasn't often and I always had to tell them to keep them informed - again, at no time were they judgemental.
I've waffled somewhat and missed a few things out but for me, not having to stress about money while trying to get my head around my gambling problem was massive and I know I wouldn't be in my current position now if not for them...