This is why they call pen 'Biro'

Before 1938, the usual style of pen in use was the fountain pen.

    A fountain pen.

A fountain pen according to Wikipedia is 'a nib pen that contains an internal reservoir of liquid ink. The pen draws ink from the reservoir through a feed to the nib and deposits it on paper via a combination of gravity and capillary action. Filling the reservoir with ink may be achieved manually, via the use of an eyedropper, syringe or a vacuum to transfer ink directly through the nib'. 

The pen was efficient but has liquid ink which takes some time before it can dry up on paper during usage.

There was need for improvement and in 1938, a Hungarian named László Bíró sold firstly a new type of pen that has a thicker ink that could dry faster on papers, the 'ballpoint pen'.

    A ballpoint pen.

A ballpoint pen according to a dictionary 'is a pen, similar in size and shape to a pencil, having an internal chamber filled with a viscous, quick-drying ink that is dispensed at the tip during use by the rolling action of a metal sphere'.  Mr. László Bíró had noticed that the type of inks used in printing, dried faster than that used in fountain pens. As at the time that László introduced his type of pen in 1938, the Second World War which sparked off in 1939 was looming. The British Royal Air Force needed a new pen that would not leak at higher altitudes in fighter planes the way fountain pens did. The ballpoint’s successful performance for the Air Force brought Biro’s pens into the limelight. 

According to Mary Bellis, "Biro had noticed that the ink used in newspaper printing dried quickly, leaving the paper smudge-free, so he decided to create a pen using the same type of ink. But the thicker ink wouldn't flow from a regular pen nib. Biro had to devise a new type of point. He did so by fitting his pen with a tiny ball bearing in its tip. As the pen moved along the paper, the ball rotated, picking up ink from the ink cartridge and leaving it on the paper".

This singular transformation of pen gave huge boost to writing and efficiently improved writer's speed. And for this, people began to call pen 'biro' because of Laszlo Biro.