So can act as a substitute for an adjective, adverb, or a whole clause if we don’t want to be overly repetitive:
Richard was enraged at his brother’s stupidity and he had every right to be so. (= to be enraged)
Elon considered the suggestion seriously and Helen even more so. (= even more seriously)
Mike’s going to be there. At least I presume so. (= that he is going to be there)
An auxiliary verb (be, have, can, will, would, etc.) can be used instead of a whole verb group or instead of a verb in order to avoid repeating words from a previous clause.