I believe the days when backpackers were just travelling around with no plan where to stay the next night and just showing up in a hostel in the evening and asking for the cheapest bed available are kind of over. Which I believe is a pity, because on my travels this meant a lot of freedom for me. But it also happened that I ended up in London spending a very cold night in front of Victoria station sharing a cardboard with some homeless people (which was an interesting experience by the way).
So today it seems people are scared not to find a bed in the evening or are more ambitious about getting a more comfortable room or a private one because they are travelling with their boy- or girlfriend. So you need to book ahead.
I see many travellers booking their whole journey, which means they have a very strict timetable and it will be difficult to change something like leaving a night earlier or staying a night more, because this would mess up the whole trip.
Those online booking engines like hostelworld.com, hostelbookers.com or hostelsclub.com became very popular because of their convenience and the possibility to compare the places to stay very easily (like prices, ratings etc.) But you should be aware that the so called „deposit“, which is usually 10% is no real deposit, its actually a fee the hostels pay to the booking platform, because the hostels never see anything of the deposit. This money is charged by the booking website. So if you see a bed for 10 €, and you pay 10% deposit which is 1 € you will have to pay 9 € to the hostel. So the hostels gets 9 €, the booking website keeps the 1 €. What actually happens is that many hostels charge higher prices on this booking platforms in order not to loose the 10%.
Its often cheaper and smarter to make a direct booking with the hostel of your choice. Many hostels offer free booking from their own website. So just do a web search for the town you want to go and „hostel“ to get usually some good results. Some people are even smarter. They use the big booking websites to find hostels and read the ratings, then they search for this specific hostel name and make a direct booking. Direct booking also have the advantage that you can also ask questions to the staff, get more detailed info about the place you want to stay at, you will many times not need a credit card and you don’t pay a „nonrefundable deposit“.
Also, it can give you more freedom, because you can try to book only a few days before you go. If some places seem to be booked out in advance, it usually happens that somebody cancels and chances are high that beds open up a few days before you want to go there.