Because you are an awesome person who loves to support awesome ideas ;)

The boring answer is because support is what keeps sites like this alive. This is how support is usually defined as:

1. Traffic
2. Advertisement
3. Content
4. Donations

I understand if you choose to stick with options 1-3, and I will be grateful for it no matter which one(s) you have chosen. But if you are feeling generous - or just miss that warm feeling in your heart - then consider making a donation to the site. It will be greatly appreciated.
A warm feeling in your heart.

Oh, and the option of setting the visibility/order/state of each of the data panels displayed on the site.

More perks may be added in upcoming updates, so stay tuned.
Items from the PAYDAY 2 universe:
- DLCs
- Codes
- Marketable items (skins and drills)

Items from the Steam universe:
- Games
- DLCs
- Codes
- Marketable items (anything worth more than $1 on SCM)
Because it is more complicated than I would like it to be - there are only a few viable choices for me and they all have significant drawbacks. This decision is not set in stone though, if cash is your preferred option then please leave me a note and I will get back to you.
Contests & giveaways sponsorship, website fees payments and development/maintenance funds.
That will be up to you and/or your donation type. The criteria I use is as follows:

- Items not belonging to the PAYDAY 2 universe can not be used to sponsor giveaways.
- PAYDAY 2 items donated without any goal set will be given away.
- Personal gifts, i.e. items I am allowed to redeem on my Steam account, should be noted as such. This applies to items of both universes (Steam and PAYDAY 2).
- If you gift me something I already own I will either trade it (Steam) or give it away (PAYDAY 2).

Note: If you do not want to be credited as a giveaway sponsor please state so in the notes as well.
Yes but they will be traded for other Steam items, which in turn will be sold for cash. I will not host selective region-restricted giveaways, sorry.
Just send me a trade offer on Steam with the items you would like to donate and the donation goal in the notes (if any). If the item you are donating is redeemable via Steam code, add the code to the notes and any random item from my and/or your inventory just so the offer goes through. These offers will be declined.