Presumably you are reading these word blurbs because you're interested in learning some useful words. You can add presumably to any statement that you think is true — it indicates a logical conclusion for which you don't have definite proof.
When you add presumably to whatever you're saying, you're giving notice that you think what you're saying is true but telling your listener not to ask for the evidence. You might watch beer commercials and say to yourself, "Presumably advertisers think people will find this commercial amusing and want to run out and buy tubs of this beer." The adverb presumably is related to the nouns presumption and assumption, which are also notions you accept without proof.