who needs god when you've got me ?

"who needs god when you've got me?"
I dont understand the meaning of this sentence.
I read some like this sentence.
"Who needs a diary when I can write to you?"
"who needs a mountain when you can learn to climb a wall?"
they mean " someting is not necessary when you do/can something"?
Im confused.
also can you let me know when you use the sentece like those.
Please, help me!!!
I will aprreciate you if you help me!

Who needs God when you've got me ?

'someting is not necessary when you do/can something' :  something is not necessary if you have another thing that is adequate/ fulfils the same purpose

'when you use the sentece like those' :  when you would use expressions [sentences] like those

'I will aprreciate you if you help me' :  I would appreciate your help

The title is probably intended to be ironic :  "I'm good enough to be God"

More often/usually this type of expression would be used to suggest that something is not necessary.

It is intended to be ironic  i.e. make a statement humourous [amusing] by turning it in to a question to which the answer is already given/provided, or rather 'implied'.  For example, your [good-looking] boyfriend might say to you  "Who needs Leonardo di Caprio* when you have me ?"  i.e. I'm as good-looking as he is.

* make your own choice of 'icon'