Advantages of using Posting key instead of master key:-
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The Master Key should be used only in absolutely necessary situations.
While in the usage that we mostly do in Stem Blockchain like voting, posting, cpmmenting, downvoting this kind of things can be done by the use case of Posting Key.
While in a little more sensitive case such as transfer funds and powering up we need Active Key or owner key/ MASTER Key.
Despite the above things Master key can be used to changed all the keys above mentioned. So if the Master key is compromised in a way a user will loose access to the account.
This key is the most important because it allows almost total control of your Steem account. The one who owns it is able to vote (for the articles and comments but especially to vote / downvote the Witnesses), to transfer your funds, to trade, to power up and power down the account, and especially, to modify all the other keys, including herself, the Owner / Master Key.
This key should only be used if really necessary and no other alternative is possible. This key is to be used when transferring funds or when a change of keys becomes unavoidable (if compromised for example).
By losing this key, it is not possible to recover your account nor the funds that are on it. It's up to you to do what you can to avoid losing it by keeping it, preferably, offline.
In general, we use its posting key when connecting to Steemit. With this key, it is not possible to make transfers of funds.
So, in this case, the only thing that the key thief can do is publish articles / comment and vote / flagger.
I don't think you should ever use your master key. Your master key is what controls your entire account.
A posting key is what you would use for 99% of everything you do. If someone asks for the master key then I would not provide it. If you hand it out you are running the risk of being hacked.
We are provided with three keys
I have only used my active key a few times but never my master key. I think when you power down you would use it or for changing your passwords. Other than that never use it and stay clear of ever providing it and you will be hack free and safe.
I don't really feel like you should ever be using you master key. I only ever use my posting key and my active key. I feel a little weird even when I have to give my active key, but I do it still because there are certain sites that require it.
If I was ever prompted with only the option to use my master key, I would probably leave that site and never go back. The way I understand it you would only ever want to use your master key if you needed to recover your account somehow or get in if some of your other keys were compromised.
Best practice is to only ever use your posting key if you are able. There are a couple that you can use your active key and it will probably be safe.
Less security risk IFthe key were ever to get intercepted and used to access your account.