I've tested the mobile application (as a tester) of HitBTC (it's not public yet), and they've done a decent job in developing it. The only thing which I hated the most in it is the accessibility of the account through API keys instead of username/password. Other than API login, everything looks perfect.
They mostly do it to earn some profits from an "arbitrage" opportunity. They:
-buy cheaper at X exchange to sell for more at Y exchange (seeing buy orders of better values at that Y exchange)
-then realize that the coin isn't even available for withdrawal through X exchange
-finally, bash that X exchange and eventually sell it back for an even cheaper price; they ultimately get into a little loss instead of a little profit.
Before arbitraging is a good way of earning quick profit. That was before when bitcoin transactions are around 10k sat to 20k sat. Now it is higher and altcoins too. HitBTC probably got the worst withdrawal fees on altcoins.
About $15 flat fee on withdrawing SENT from HitBTC:
Without reading up further notice but still making deposits even the wallet is on maintenance and now they blame the exchange? 
They mostly do it to earn some profits from an "arbitrage" opportunity. They:
-buy cheaper at X exchange to sell for more at Y exchange (seeing buy orders of better values at that Y exchange)
-then realize that the coin isn't even available for withdrawal through X exchange
-finally, bash that X exchange and eventually sell it back for an even cheaper price; they ultimately get into a little loss instead of a little profit.
Before arbitraging is a good way of earning quick profit. That was before when bitcoin transactions are around 10k sat to 20k sat. Now it is higher and altcoins too. HitBTC probably got the worst withdrawal fees on altcoins.
About $15 flat fee on withdrawing SENT from HitBTC: