Nationalclothing.org

Lederhosen or bundhosen are short leather trousers embellished with embroidery and traditionally used in Germany. Usually, the knee-length breeches are called “bundhosen”, while short trousers (mid-thigh shorts) are called “lederhosen”.
Lederhosen and bundhosen traditionally have a kind of suspenders and a drop-front flap. You can see the drop-front flap on the photos: it is embroidered and buttoned with 2 buttons to the main piece of the breeches.
Traditionally lederhosen and bundhosen are made from leather, but a fabric can also be used.

What is this "drop-front flap" on lederhosen is for? Something like intimate needs (toileting and so on)?

As far as we know, yes. The invention of this kind of flap was a sensation, and it quickly became very popular. It's like a modern fly - very comfy for men. Some people think that Bavarians invented the drop-front flap.
There are some old lithographies that picture men in trousers with a drop-front flap urinating or having sex through the opening.

A little bit of history

Lederhosen literally translates as “leather pants” in Germany. Using fur or animal hide for pants goes way-way-way back to Ötzi – the Iceman, the 5,300-year-old frozen mummy who was found near the Alpen-Austrian-Italian border in 1991. But lederhosen, as we know it today, first took shape in the 1700s. Lederhosen fell out of fashion for a time in the 1800s as pants made from cotton or cloth began to gain popularity. But when King Ludwig II, who was Bavarian, professed he was a fan of the style, that was that, and lederhosen were back. Farmers and aristocrats alike started wearing lederhosen.