As far as I know,

1) Same as it has always been, used to reward Storage Node Operators for sharing their free storage, that means the storage is paid in STORJ by the network itself, anyone else developing on it may accept other type of currencies and crypto however the storage itself is paid in STORJ so there will be more demand for tokens as the storage usage grows. It also offers discounts to end users paying for their own storage in STORJ. I also gathered that the current and future Open Source partners may get some revenue sharing for using the network with their own software and tools. Not sure if it's revenue sharing in STORJ or not, but here's how it reads on their own blogpost:

https://storj.io/blog/2018/10/open-source-revenue-models-in-the-cloud-era/

"The Storj platform can generate a sustainable revenue stream for open source projects every time their code is used on the network. As decentralization continues its rise to support new web and technology infrastructure, these mutually beneficial opportunities shouldn’t be overlooked."

2) Not sure about your incentive, in my case I hold the token for future use on the platform, I also believe the project will do well in the future seen all the new partners and their own projects. If all goes well more storage usage means more demand for the token to pay for the actual storage as the only ones offering storage are Node Operators (farmers).

3) So far they have accepted ETH, BTC and STORJ not sure what the plans are for FIAT but I believe they used Stripe in the past. As I mentioned above whatever they accept from customers, developers and corporations will have to be converted to STORJ to pay for the storage on the network side.

Hope this helps. If you join their community you may get more precise answers.