Stonewall is basically a "what you see is what you get" defense. On the positive side, you get a very solid formation, a safe king, and opportunities for counterplay which depend on the opponent. On the downside, a huge hole on d5 and a terrible bishop (the light one).
So it's an aggressive defense, but not a real mobile one. There are certainly other Dutch variants that offer more interesting play with the same security (and often some of the same defects).
As to the "reason" behind playing them, the idea of f5 is always to gain kingside space, usually allowing your opponent to play on the queenside. It's a little bit like saying "Hey, you want THAT crappy queenside? Have it. I'm building over HERE on the important part."
White, against f5, can play e4 as a gambit and it works really well to open things up, makes Black's position more passive than he might prefer.
If on the other hand you seek a more active style of play, less positional sounding-out and more dynamic piece play, then any number of other defenses might be a better choice - Grunfeld, King's Indian, Queen's Gambit Accepted etc.
I've been playing chess just over a month and have progressed at what i believe is a rapid pace. Something has clicked in the past 4 days or so as im seeing the board better and competing and winning against players in the 1300+ range despite my 1100+ rating and I figured it was time to get into learning defences both for and against me and I've come across stonewall and dutch-stonewall and I see that they have good reputations and they fit into my rating/skill level and that of opponents...the little I've found about this/these is that I can do virtually nothing once the first four pawns are laid out regardless of the opponents moves...this is all without getting into an in- depth understanding of the root why's of their origin/reasons for their existence, etc...basically I don't want to read a book as sites have suggested and would rather not read pure stereo instructions/notions I would like prose in more human terms to maybe understand this OR ANY suggestions/opinions(on this defense or another style) as I feel I need to morph my current play into something more tested/true as I doubt at 33 I'm going to turn the chess world upside down with my bullet knights forking yer rook and king and turn this into the Dallas Defence that will be copied for years to come...thanks in advance for any help you may be able to offer-Matthew Marshall:)