No matter how great your team is, no one can be consistently creative. By partnering with a fashion influencer, you'll have a chance to work closely with someone who's an outsider to your brand. They can have a completely different approach to communication with their audience and creativity. By working closely with them (and working closely is something you should strive to do), you'll be able to reignite internal creativity too.
This is one of the things that make influencer marketing much more powerful than brand ambassadorship, which is very popular with big fashion brands. Ambassadors are usually celebrities. Most of the time they don't have a say in the planning stage.
On the contrary, influencers have negotiating power when it comes to campaign creation and planning. They will create content that aligns with your guidelines but when given freedom, they can bring a unique perspective too. In other words, a brand and an influencer are equal partners.