There are many factors at play...
as far as I know 10/100 is slower than AC for a close range. Am I right?
All things being equal, yes. But wireless, even 802.11ac suffers from issues with retransmission, signal strength, etc.
If I want to send some files from my wired computer to my phone, the
wired part will be the limit, not the wireless
The limiting factor would most likely be your phone and computer's HDD transfer rates, assuming you are getting 100Mbps at a minimum on your network connectivity.
If it is aimed at people who only use wireless devices, why does it
have 4 LAN ports?
Other devices, such as wired printers, VoIP phones, a possible wired computer, cascading switches, etc.
So why not 10/100/1000 wired ports? Choice vs. cost. Most people will never need a gigabit LAN at their home...so choosing the slightly cheaper option makes sense.
Even 802.11ac buyers tend to often be misled. They focus on "wow that's fast" and forget that they bought a 10/1 internet connection or that they never share data between the wireless devices and are only doing "hub/spoke" connectivity to the internet. In those cases, as long as signal strength is still decent enough even an 802.11g router would suffice.