What is the meaning of 是 here

I came across this sentence and a couple others with a similar structure and I couldn’t quite figure it out. 她一定是忘了他的承诺了, the verb 是 is used here but, as I see it, doesn’t really contribute to the sentence which translates to “She must have forgotten the promise”. Is there some other function to 是 I wasn’t aware about? I asked my girlfriend who is a native speaker and said the sentence would still make perfect sense without it but couldn’t really say what 是 added more to the sentence. I hoped you guys could help me figure this out. Thanks

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