It's not that. I have switched jobs now so haven't got a PA in the same way but it just enables you to get on with your correct job and keep focus on what you are meant to be doing. And some is junior development.
Please print these docs (often sent when I'm on the phone to someone else) means please print, deal with the printer running out of ink / paper /, hole punch or staple them how you know I like them. Ideally put them in a file for me (again how you know I like them because we work together). Often I might ask for something to be saved to the system with the correct protocols and links sent back to me. Because that's what I need with all metadata stripped because you know that's how I want it.
Please book a train ticket. Means do it in accordance with company policy. If you use my card fill out my expenses claim for me and arrange for it to be submitted. If not, look up the times, arrange for payment, arrange it to be attributed to the correct file. Book me a taxi to the train station. Arrange for tickets to be delivered to my desk or if e-tickets to be sent to my phone. Check with me I have everything I need.
Please call so and so and say xyz. Actually, I might not have time to speak to them because every time I speak to them they want to talk about other things so a 5 minute conversation turns into something else. Maybe I am late on getting something back to them and don't want to have that awkward conversation. Maybe they have asked me something complicated and I don't know the answer yet. Maybe they never answer the phone and I haven't got time to be faffing around trying to speak to them. Maybe I'm head down in something else and whilst I can ask you to call I haven't got time to completely switch focus and call them myself because I can be internal me with you but have to be external me with them. Maybe as a matter of presentation they wouldn't expect me to be the one calling about something trivial.
Maybe it's a way of introducing you to the other party so you can be more involved in the future.
Have no fear though, developments in IT mean that PA jobs are being phased out so everyone will just have to get on and do it themselves and you won't have to do it. I don't agree it makes us more efficient though - just occupied doing something else. My job means I have to record my time and whilst it used to be easy to get 7-8 hours of chargeable time straight day in day out without working particularly long hours it's now much harder as I spend so much time faffing about with stuff like this.