If your website is getting good traffic but low page views or high bounce rates you need to review your legal website and look at what types of engagement tools you can utilize to keep people on your site. If your site accurately depicts the strengths of the firm, is easy to navigate and is easy on the eyes, video is a great tool to increase the amount of time a prospect stays on the site. You also hit different learning styles (think generational gaps here) when you incorporate video - not everyone wants to read your legalese in depth - they want the information they came looking for and they want it quickly.
Video can also humanize and personalize the attorney or the firm with a prospective client, specifically around sensitive legal issues (divorce, custody, criminal defense, sexual assault, etc). A video can help build that relationship before the prospect picks up the phone. My recommendation here is to do a firm overview video on the home page and get into more specific topics on practice area pages and have a video library accessible from your home page.
If you choose to do video there are a two important things you want to consider. A video has to be well done to be effective - skip the iPhone "selfie" video and get some professional help. As much as good video can enhance your image and keep people on your site, a bad video can send people screaming from your site. If you choose to get video done professionally, make sure you get the rights to the video so you can distribute as you see fit. I would also recommend working with a service that will help distribute the video. It is one thing to have a nicely done video but you also want to have people see it!!
There are also several SEO benefits to video if done correctly. Video should be transcribed behind the scenes so you get the keyword benefits. Video thumbnails also can appear in search and will often get better click through rates than search results that are higher.
At the end of the day, if you need people on your law firm website longer or if you work in a sensitive area of practice or your target demographic is younger, video can be an effective marketing tool for a law firm.