Platform ticket is a ticket which is sold at each station in Japan(if the stations are staffed or equipped with automatic ticket machines) which allows the bearer to enter the station beyond the ticket gates and access to the platforms but not allowing the bearer not to board any train.
May be many people will wonder who will buy this ticket. Not like in continental Europe where most train stations do not have ticket gates, middle size or larger stations in urban areas of Japan are staffed and have ticket gates which do not allow anyone without ticket to enter the platform. Therefore, tour guide or tour assistant who has to pick up or see off at the platform has to buy this platform ticket if he or she does not have a commuter pass of that station. Also it is common to buy platform ticket when railfan wants to reach the platform to take pictures of trains.
Platform ticket is valid only on the date of issue, which means not able to buy in advance and duration is up to 2 hours from the issuance. Please be aware that the duration is not from the moment you enter the gate. If your ticket is issued at 13:34, you must get out the station till 15:34. Therefore, do not buy the ticket first and take pictures of exteriors or public areas of the station.
By the way, if you enter the ticket gate by smart card such as SUICA or PASMO, you can not get out from the ticket gate but have to go to the staffed gate and the staff will deduct the amount as same as for the platform ticket.