why we use were for past tense?

In past continuous tense when subjects are plural (We, They,You,Dogs ) we use "were" For examples :- You were driving your car. They were eating their food. We were planning about our studies. Dogs were barking.

In simple past tense when there is no verb in the given sentence then we use "were" as a helping verb with we,you,they, more than one name, plural subject. Ex:- They were poor farmers.