Apart from your travel documents and clothes, backpackers like you also need a lot more little things as additional reinforcements. Like...

Rain jacket. It's light and compact. Since you will be walking a lot to save on transport cost (and to discover more of the local scene), it's always best to have one to protect you from the unpredictable weather. This can also be used when you need to pack something wet, along with your dry stuff.

Plastic Bag. For waterproofing your stuff, should it be needed.

Swiss Knife. With all the possibilities it makes available to you, you'll definitely find a use for it.

Sleepsheet. Should come in handy when that cheap hostel you booked scrimped by not hiring a cleaning staff.

Padlocks. You definitely do not want to lug around eveything all the time nor would you be happy if your stuff gets stolen.

Wet Tissues or AlcoGel. Keep those Hands Clean and the germs away.

Earplugs. To block out the snores coming from that guy lying on the bed next to yours.

What else did I forget?