Resistor arrays

I'm in the process of building a logic circuit that called for the use of a resistor array.
There was no BOM so I was left to try to work it out myself.
What is a resistor array & why would you use one?

They appear to be a network of fixed resistors set to a specific value.
The value can't be changed.

As Ive discovered you can also have arrays of diodes & capacitors. !!! Wow !!!

Above is a resistor array. Below is a diode array.
Notice they all share a common connection.

Usually, they don't come in these formats.
They come in packages like this:

There is usually a dot at one end which indicates the common terminal.
They come in many different configurations.. series, parallel, etc etc.

There is a code for identifying resistor network values:

I just purchased the A09 104 on ebay. This is a series array of 100K resistors.
A09 : A=series, 09 = 9 pins, 104 = 100K

Resistor arrays are very useful if you need a network of resistors all connected to one piont.
(EG: 8 pull-down resistors, each connecting microcontroller pins to a common ground).

So if you can't find the array you need, it looks like you can build it, if you know its specs..