"Only seat reservations" option

It's a useful option if you already have a ticket and only want a seat reservation.

It is also useful for railpass holders (at least on Railjet trains) where you want a seat reservation. Although not required, can be useful on popular trains if you want to be sure you will have a seat and not have to stand. Also available on DB long distance trains and certain Swiss trains, where many people travel with passes.

For example with Railjet, how come you wouldn’t want to pick your seats at the time of purchasing the train tickets? Why come back at a later date to now reserve seats, which might be taken?

You could be travelling through Europe on an Interrail pass and only want a seat reservation since you already have a ticket.

You could be on a business trip with a flexible ticket when the last meeting of the day gets cancelled, allowing you to take an earlier train home and you want a seat reservation on that train.

You could be a commuter travelling with a monthly ticket of some kind that want to buy a seat reservation so that you don't have to walk through the train in the morning looking for a free seat.

Thank you. Makes more sense now.

Basically when it comes to train tickets there are two types: IRT and NRT.
IRT means "Integrated Reservation Ticket". Such tickets always come with a reservation.
NRT means "Non Reserved Ticket". Such tickets do not come with a reservation by default.

Think: IRT means I have a ticket for a train, NRT means I have a ticket for a route.

Latin Europe is mostly IRT, whereas Germanic Europe mostly uses NRT. And where tickets and reservations are sold separately you do indeed have this separate "reservation only" option on the website.

Another important difference between IRT and NRT is that with NRT tickets you can usually reroute yourself at your own discretion when there is a disruption. ("Just hop on another train"), but with IRT you normally need to have your ticket exchanged in this case.

"For example with Railjet, how come you wouldn’t want to pick your seats at the time of purchasing the train tickets? Why come back at a later date to now reserve seats, which might be taken?"

One issue people have been encountering recently is that you could buy tickets months in advance, but could not yet get reservations, as OBB is shuffling trains around, and didn't know yet what rolling stock it was going to use to operate a service. So you bought your ticket early to get the good price, and then would come back later to get reservations.

And then there are of course people who do not want to pay the extra fee for reservations, and just buy the ticket, and plan on looking for a free seat. It is rare for all seats on a train in for example Switzerland or Austria to be taken.