Long transistors are split into smaller ones for various reasons.

One reason is to fit them better into the overall layout of a block or simply to have a better aspect ratio.

Another reason is to reduce the gate resistance of the device. The problem is that the gate-channel capacitance forms a lowpass with the gate resistance and switching speed is reduced. In addition the drain area is reduced because two fingers share a common drain and therefore parasitic capacitances are reduces as well.

Splitting a transistor with some W/L ratio into a transistor with multiple fingers is done such that the width (W) stays the same and the length of finger is L/k, where k is the total number of fingers. The effective length and width stay the same, but the gates are now in parallel and therefore the resistance is reduced. In addition, gates are often contacted at both ends to reduce the effective resistance.

Splitting transistors can either be done by using multiple transistors with a single gate or with transistors that have multiple gate fingers.

Transistors with multiple fingers have the disadvantage that the current direction is different for two neighbouring fingers. E.g. if for the first finger the source is to the left then the source for the next finger will be to the right. The properties of transistor can change depending on the current direction. Therefore extra care has to be taken when trying to achieve good matching.

Using multiple fingers to obtained scaled current sources for example in a current mirror is also considered inferior to having multiple single gate transistors because of slightly different properties.

An analog designer therefore has to decide which option is the best for a given problem.