Although not relevant or even possible for every trade, if it is, getting before and after photos makes for an impressive addition to your website.
Having any form of gallery is good practice as it allows potential customers to actually visualise what it is that they'd be buying, but with before and after pictures, you add a degree of relatability. Whether it's a dirty oven, an old fashioned bathroom or a dilapidated property, people with similar ovens, bathrooms or properties will relate to those before photos and desire the after.
The secret is to get into the habit of getting before and after photos on every job. Not every set of photos will end up looking that impressive, but the more you take, the greater the chance of getting something that will really wow!
Some tricks to getting better pictures:
Just remember that before and after photos are proof to potential customers that this can work for you, too. If you're not already using them, start immediately.