I use TripIt as the foundation for all of mine.
- You can subscribe to your TripIt calendar, so that all of your trip details automatically show up on your personal calendar app. You only have to do this once (not for every trip). If like me, you use your calendar to run your life, this is a huge productivity benefit.
- You can forward your flight confirmation emails and accommodation booking emails from most hotels, trip booking sites (e.g. Booking.com, Kayak, Amex Travel) and even Airbnb/VRBO/etc. to TripIt, and they parse the contents and add the details to your itinerary
- For me, it's now become my source of record for when I was last in/out of the country, which I find super useful for US immigration stuff (green card, global entry, citizenship applications, etc.) and some non-US visa applications too
- If your company uses Concur for travel/expense management, you can link your work account as well so that work trips show up there too
- I think this might be a Pro feature, but you can get alerts of gate changes, flight delays, baggage carousel assignments, etc., oftentimes even before your airline informs you
- I don't use this as much, but you can also invite people to individual trips and they automatically get all itinerary updates
There's lots more benefits, but these are my top ones that come to mind. The UI is super old school, which I don't love, but the convenience far outweighs this and some of the other cons for me.