Port-group explanation

Can you please give me an explanation (for dummies) about port-groups? 

Why do they exist? When would I ever need to use them?

A relative document says: "port-groups determine the available front-panel Ethernet ports and supported breakout interfaces" 

For example, the S4148U-ON switch has the following port-groups:

Port-group Mode Ports FEM
port-group1/1/1 Eth 10g-4x 1 2 3 4 -
port-group1/1/2 Eth 10g-4x 5 6 7 8 -
port-group1/1/3 Eth 10g-4x 9 10 11 12 -
port-group1/1/4 Eth 10g-4x 13 14 15 16 -
port-group1/1/5 Eth 10g-4x 17 18 19 20 -
port-group1/1/6 Eth 10g-4x 21 22 23 24 -
port-group1/1/7 Eth 100g-1x 25 -
port-group1/1/8 Eth 100g-1x 26 -
port-group1/1/9 Eth 100g-1x 29 -
port-group1/1/10 Eth 100g-1x 30 -

Why would you need port-groups to find out the supported breakdown interfaces???? Confused.

Many thanks