Virtual field trips help to connect students to their learning by immersing them in the environments that they are studying. This can help to increase reading comprehension and content memory. Virtual field trips also allow students to see and experience places that they may never get to in real life: from mars to the ocean floor.
Using technology to "leave the classroom" through a virtual field trip causes all the problems technology can normally cause however. Students may wander to other websites and play games or watch videos. It also makes lessons reliant on technology, so if there are technological issues the lesson is unusable. It is also important to make sure that virtual field trips do not become a complete replacement for real field trips. While they are useful, they are nowhere near as engaging as actually getting hands-on with material in those places.
To get students the best experience from virtual field trips you should first make sure the material is age appropriate. Even if your fourth graders are learning about space, not every space video is appropriate. Resources should be engaging for your students specifically. Second, find a way to keep students attention on the field trip. These experiences are great, but remember to have an assessment--this can be as simple having a conversation after the video or exploration. Finally, don't overuse the strategy. The content retention may not be as strong if virtual field trips are the primary method of learning, so pair them with articles, slide shows, and projects.