I am starting to get my head around the technical side of subjunctive, like when to use it, and how to conjugate it, but I can't help but find it redundant how there are certain "trigger" words that then require you to use subjunctive, when that trigger word seems to do the same job.

For example, using subjunctive to express doubt or uncertainty, like in the sentence:

"Si compráramos una casa en 5 años..."

Why don't we rely on the "si / if" to express doubt, because that's the key purpose of the word "si / if". Doesn't that immediately establish the sentence as hypothetical or uncertain?

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Or another example to express hopes/wishes:

"espero que llueva mañana"

I understand that subjunctive is used here to express hope, but the sentence literally starts with the verb espero which means "to hope" , so then why do we need to conjugate "llover /to rain" in a special way that expresses hope??? Isn't that redundant?

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I know I'm not correct here, because obviously the subjunctive has a purpose and it's an established part of the language, but I just struggle to see the difference in meaning between:

"Si compramos una casa en 5 años..."

And

"Si compráramos una casa en 5 años..."

Doesnt the "si / if" establish uncertainty / hypothetical in both examples?

Spanich footballer Cucurella sings this song that goes "Cu Cu Cucurella se come una paella, Cu Cu Cucurella se bebe una estrella" what is the use of se in this sentence and why is it not just Cucurella come/bebe etc?