Transistors in LED matrices?

I'm confused about the purpose served by using transistors in an LED matrix.  Why not simply use current limiting resistors and a shift register?  What purpose do the transistors serve?

Re-design6 years ago
I don't know what circuit you're talking about but in a  circuit like this one, it allows you to control more lights with the ic than you could without the transistors,  The transistor uses a small current to control a large load.
Hello, what you say it's clear, but I'm still asking myself: what rules I have to follow to choose the right transistor? I mean, if I want to light up at the same time 5 leds or 20 leds, I can use the same type of transistor?
In this case it acts as a switch that can pass more current than the ic can.

You have to calculate how much power you are drawing with the lights and then pick a transistor that has a higher rating than that.
Ah ok! That's clear, thank you! :)