There are many answers, some positive and some negative, to your question.
In my opinion, though, the reason is that the world needs more than a single lingua franca, other than English - because despite it being extremely easy to pick up and master, it does not adapt to the people that use it, but rather the people themselves have to adapt to the language. Not everyone can think in English. Esperanto aims, even if very loosely, at filling that semantic gap between different European languages, which is more than what English did with their loan words.
A secondary explanation is that it's just a language that will pick up steam immensely, probably to the point that it will be the fourth most spoken language in the world - as many Chinese and parts of its government expressed interest in using a language that is much easier to learn and get their point across with foreigners.
Then again, you may as well see these points as negative, as a conlang is only as useful as its users make it to be and how accessible it is to you.