Why to include:_spf.google.com ?

Hello,

if i understand correctly, at https://support.google.com/a/answer/33786 is suggested to delete my existing DNS TXT SPF record in my hosting account and create new with value "v=spf1 include:_spf.google.com ~all"
I do not understand why. I thought i should only use hosting server IP.. By the way, in my current SPF record i am also including hostname of the SPAM filtering system of my hosting provider (necessary for them to relay my e-mail out).
Why i should be adding google...

Why to include:_spf.google.com ?

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