Whereas up until a few years ago tourists used to be able to pay for many things directly with USD, nowadays almost everyone in Myanmar demands Kyat for payment. Combined with the reality that Myanmar is very much a cash-based country with credit cards almost never accepted, you need to either bring USD (or Euro) with you to convert to Kyat, or make use of ATMs to withdrawal cash Kyat
In recent years ATMs have become more widely available in Myanmar's major cities and tourist areas (Yangon, Mandalay, Bagan, Naypyidaw, Inle, airports), to the point where you don't really need to convert USD anymore if you don't want to
However, bringing USD to convert for cash can still get you better rates than using the local ATMs to withdraw cash
ATMs are almost all affiliated with banks, and they work with most international bank cards. KBZ is the most common ATMs I've seen around the country
ATM Fees: 6000 kyat per withdrawal - $3.9USD
Max withdrawal: 300,000 kyat per withdrawal - $195 USD (some also max out at 200,000 per withdrawal, which is just $130USD)
Additional fees: your own bank will likely also charge you a international withdrawal fee on top of what local ATMs will charge you. This varies by bank so check with them first
Exchange rates: very competitive, almost no spread on the exchange rate charged by the ATM vs. the market rate (at least on the KBZ ATMs I used)
Bottom line: depending on your home bank, you're looking at a 2-5% total fee for making ATM withdrawals in Myanmar. Cash exchange rate in Myanmar with USD for Kyat is usually about a 1% spread, so if you're able to obtain USD with your home currency for less than 1-2% spread, it's better off to bring cash instead of making ATM withdrawals