Why is it sometimes okay to use different verb tenses?

Different verb tenses are used to convey different meanings. For example:

We use present simple to talk about something we do again and again ie habits.

I usually go to the cinema on Saturday.

We use past simple to talk about something that happened in the past and it is now finished.

Last saturday I went to the circus.

We can combine these two sentences to say:

I usually go to the cinema on Saturday but last Saturday I went to the circus.

We don't usually talk/write in just one tense