Perhaps intermediate and higher levels than that. Indeed an English-English dictionary is useful when you don´t find a word in your mother tongue (in the bilingual dictionary you have). Or when you want to "force" yourself to get acquainted with the way, natural way to say something in English.
In a monolingual (English-English) dictionary you also will find day-to-day and updated examples of sentences and word usage.
So, it´s complementary to your English-Portuguese dictionary, and a way to build up vocabulary; along with a trove of information of a given word.
But just don´t throw away your Portuguese-English dictionary, as I said, they can be complementary and do a lot of good to your English.
Not to mention, other complementary reference materials, like dictionary of "idioms", etc.