Some of the localisations in use for MediaWiki need improvement. It can be for all kinds of reasons including bad grammar, typos, spelling errors but also because a message changed. When you are an administrator on a wiki, you can change such MediaWiki messages on your local wiki however, you should not.
When you improve a message locally, the result will be local and it will override the messages that come with the MediaWiki software. This includes the daily messages that are new or have been updated and come to you through the LocalisationUpdate process.
When you improve a message at translatewiki.net, you will find the experience much more productive compared to working on your local wiki. Tools are available to you that suggest translations, help you proofread what is already there. If anything, translatewiki.net exist to make you as a localiser as efficient as possible. You will find once you get going, you are at least ten times more productive.
For some people localising on-line is what prevents them from being productive. When the local connectivity to the servers of translatewiki.net leave a lot to be desired, it is sometimes better to work off-line.
Getting the message out why you should localise at translatewiki.net has its answer in this way. However, there is a reason why some messages need to be changed locally. It is because the message needs to be adapted to the local wiki. They may be about policies, contact information or whatever else is not standard. Only a few people need to bother with this and they are the local admins of a wiki, they are the ones who can change messages locally.