2. It's A Good Way To Go Deeper On The Already Existing Universe
We already had established on the current DC universe that LexCorp is one big company that is pretty much in charge of most of the business in Metropolis (or at least, that's what I assume considering that in the film the LexCorp logo appears like 3 times). We also know that Wayne Enterprises will definitely be another big company that will most likely help rebuild Metropolis. That's all we know so far in the current DC Cinematic Universe, and if there's something we've learned with the films of the MCU is that you will need more than 2 hours to introduce and explain new elements to the story (The Tesseract, for example was teased in 3 films before it was fully shown in The Avengers). If they include Arrow in their current cinematic universe it will be much easier for them to explain many of the events that will happen between movies. For example: One of the biggest problems many people had with Man of Steel was the destruction that was caused on Metropolis as well as how things got "solved" rather quickly. If something like that happened again, maybe an interesting plot point to be developed in Arrow would be the reconstruction of Metropolis. That way they can explain those situations without messing around with the continuity of the films.