I can't speak for everybody, but there are several reasons to purchase assets (no particular order):

Quality - paid assets are typically better quality than free

Quantity - you can usually get more variety in individual pieces on paid assets

Consistency - it can be difficult to find "matching" free assets

Specificity - it can be difficult to find specific/unique pieces among free assets

Exclusivity - if you want assets that nobody else can use....

Convenience - scrolling through thousands of free assets takes a LOT of time

Patronage - some people buy assets to support artists and/or encourage them to release more (sometimes free) work

disclosure: I've purchased on GameMaker market, Unity Asset Store, GameDev Market, and Graphics River

After developing my idea, doing some of the art and relying on a very skilled programmer, I gotta say I would love to be able to buy a portion ofy assetts for early demo. I agree with boom Shaka. 

I'm just now learning Blender, up until recently, I have pretty much been reliant on OpenGameArt and asset stores for any kind of modeling. I often look for them for artwork as far as 2D and sounds as well. As a programmer mainly, and trying to solo develop, any time I can save by out sourcing is great. Although, the majority of my stuff has just been prototypes, I feel that with a few of the assets I've gotten, I could release something fairly well put together. Overall, I would say it is definitely a viable option, as long as you're are producing assets that are of quality. 

Boom Shaka nailed it, although I might add:

License - If you intend to sell your game and don't want license surprises after release. Having proof of purchasing a valid license in your favor could alleviate a potential headache of someone claiming ownership and demanding royalties.

I have purchesd assests from various asset stores, as well as used art from OGA.

I would have moral issues selling on assetstores, since the market targets primarily dreamers, meaning people who will never make real use of it and even if, they will end up as assetflips, looking all the same, flooding the market with low quality products.

So however you turn it, it ends quite bad. If you are a serious game developer and cannot make assets you probably should comission an artist to do it, it is more expensive than asset store, but still affordable and you get unique content, that fits you.

I don't think there's anything wrong with outsourcing some textures and basic scenery, but just be sure that it's mixed up nicely while also seeming consistent with theme and quality.  So, don't just use textures all from the same set all in the same place, and don't have one cartoonish "hand-drawn" texture sitting right next to photosource realism.

I don't think people will mind seeing a familiar texture here or there, or even a familiar tree or trashcan, etc.  This in itself doesn't count as an "asset flip".  If it did, then we could say the first STALKER game was an asset flip, which is something only the most obnoxious and misinformed would try to claim.

Just be sure that your characters, weapons/items and everything that is at the core of the design is original, and nobody should lose any respect for the effort put into the project.