When we looked at options for entry into the Belem sites, we compared the Lisboa Card, the Ticket Cais da História and the Monastery/Tower combined ticket. For 12 euros for an adult, you can buy a combined ticket to the Monastery and the Belém Tower. For 25 euros for an adult, the Ticket Cais da História buys you entry into the Monastery of Jerónimos, Belém Tower, National Archaeological Museum, Museum of Popular Art, National Museum of Ethnology and National Coach Museum. For 18.50 euros for the one day pass, the Lisboa Card gives you free access to these sites, unlimited public transportation and, at least according to the Lisboa Card website, the ability to skip the queue. We've bought city cards in other cities, and have found that even if they didn't turn out to save us much in entrance fees, we were always able to skip the line ups which was a significant value when you want to spend your time seeing something instead of standing in line. Here it looked like we could save some money (we definitely wanted to visit the Coach Museum) and skip the lines by buying the Lisboa Card.
We went with the Lisboa Card. There was no option to buy the combined tickets in advance. The Lisboa Card we could purchase a voucher online and exchange at a tourist information office next to our hotel.
We did get free entry in to the Tower of Belem, the Coach Museum and the Monastery and free transit to Belem (and around Lisbon later that night). We did not get to skip the lines.
With the Lisboa Card you stand in the same line as if you were buying a ticket. Worse, you must do that at each place you visit as if you were buying an individual ticket to each sight. We stood in line at the Monastery for 55 minutes to get inside and we were there before it opened. At the Tower of Belem, it was over an hour to enter. It was 33 celsius that day and neither line has any shade. While we were standing in line, we got to watch the people who had bought one of the combined ticket options skip the line.
If the value of the Lisboa Card to you is to skip the line, you won't get that in Belem.