Why do we connect a resistor before a Zener diode?

 I answered this SE EE question just now.

OP asked: "Why do we need a resistor in a Zener diode circuit...?"

We can answer this question by looking at the problem from a few points of view:

"Dynamic resistor"

From this functional point of view, a zener diode can be considered as a non-linear resistor that has the property of keeping the voltage between its terminals constant as the current through it varies. It does this by changing its "resistance" depending on the current. This can be illustrated by a simple experiment:

Imagine that a current I flows through a variable resistor with resistance R so a voltage drop V = I.R appears across the resistor. Our task is to keep this drop constant.

We can do it by a simple trick - when the current, for example, increases, we decrease the resistance to the same extent and vv. As a result, the product of the two quantities (the voltage drop) will not change.

The role of the constant resistor in this arrangement is to convert the input voltage into current flowing through the "dynamic resistor" (Zener diode).

"Dynamic voltage divider"

From another functional point of view, the network of the two resistors - constant and varying, can be considered as a "dynamic voltage divider" that has the property of keeping its output voltage constant as the input voltage varies. It does this by changing its ratio depending on the input. This can be illustrated by another simple experiment:

Imagine that an input voltage Vin is applied to the voltage divider so an output voltage Vout = Vin.R2/(R1 + R2) appears across the resistor. As above, our task is to keep this voltage constant.

We can do it by another simple trick - when the input voltage, for example, increases, we decrease the divider's ratio to the same extent. As a result, the product (output voltage) of the input voltage and divider's ratio will not change.

The role of the constant resistor in this arrangement is to make a voltage divider with the "dynamic resistor" (Zener diode) in series.

"Dynamic current divider"

When a (varying) load is connected to the Zener diode, the network of two resistors - dynamic and load, can be considered as a "dynamic current divider" that has the property of keeping its voltage constant as the input current varies. It does this by changing its ratio depending on the load resistance. This can be illustrated by another simple experiment:

When the load resistance, for example, increases, we decrease the dynamic resistance and vv. As a result, the output voltage does not change.

The role of the constant resistor in this arrangement is to convert the input voltage into current supplying the "dynamic current divider".