Especially for larger projects in multiple languages, it can be quicker to receive translations back from an agency rather than from internal teams. These are sent back neatly, with all language versions sent back at the same time within a set deadline. Internal staff have their own job roles to do at the same time, while professional translators are assigned to each project with no distractions.
Professional translators and agencies use professional software to speed up the translation process and ensure accuracy. This software also keeps a 'memory' of what has been translated, which makes repetitive translations a lot quicker and easier.
Although it may seem like an agency would be expensive, it's actually quite reasonable. And costs can reduce if you have a lot of repetitive content and volume- due to the point above.
You and your internal teams often know what terminology works best for your industry in English. And you may also be aware that these may have a 'funny' translation in French, German etc. You don't need to worry that an outside agency won't get your terms, as any issues will be clarified with you along the way.
Professional translators are just that - professional. Good translators are trained in making sure translations are the very best they can be without reading like a translation. They are also good at spotting any problems in the original text and/or translation, which makes the standard of job done very high.