It can often be confusing to traders who are using multiple futures exchanges with different contract types -- inverse and qunato, dailies or quarterlies, 5x or 100x -- so people wonder: what should I trade? Well, it depends on what your goals are. If you are trying to do a little hedge for a medium term, you would want to use a quarterlies expiration contract instead, because otherwise you'd have to reopen shorter-term contracts after they expire and settle.
When cash markets are not functioning well, cash and carry arbitrage (and its reverse) futures markets may make the underlying asset accessible to more people. It is possible that A is bullish on bitcoin, but does not wish to go through the hassles of creating a wallet and storing it safely. At the same time, B might be comfortable with bitcoin wallets, but might be unwilling to take bitcoin price risk. Then B can buy bitcoin spot and sell cash settled bitcoin futures to A; the result is that A obtains exposure to bitcoin without creating a bitcoin wallet, while B obtains a risk free investment (a synthetic T-bill). Similarly, suppose C wishes to bet against bitcoin, but does not have the ability to short it; while D has no views on bitcoin, but has sufficient access to the cash market to be able to short bitcoin. Then D can take a risk free position by shorting bitcoin in the cash market and buying bitcoin futures from C who obtains a previously unavailable short position.
Margin Call - when you run out of bitcoin on your account to cover the contract positions you have taken (your account value doesn't cover the Maintance Margin) you are officially rekt. Some exchanges like CryptoFacilities will merely notify you that you have to deposit more or reduce your positions to increase your margin. Others like BitMEX will liquidate you at this level where the exchange takes over your position to liquidate it into the market to prevent system loss.
In this guide we don't want to deal with social loss risk, so use the FCA-regulated, London-based bitcoin derivatives exchange CryptoFacilities. They offer contracts with 2% margin requirement (50x leverage) as well as a 15% margin requirement (6.5x leverage). They are 100% bitcoin based, but they don't accept US customers. Sign up here to get started: