The reason they give it to people online is that, because they've already paid, they can't not go to the park if it's snowing or raining or closed- and Merlin already have the money. This means that they will be in the park, despite the weather, but will also spend more money of waterproofs, ponchos and hot food. Therefore, booking online gives Merlin more money.
People don't book online either: because they are not aware of it, they don't have Internet access/credit card etc. (unlikely), they have a MAP or they go on the spur of the moment, and didn't know they were going the day before.
However, the stupidity of this shows when they get to the park half an hour before planned, and see the people enjoying ERT, and being able to get into the park easily- as the already have the tickets- and they are jealous of that. Even if you don't want to book online, it is much cheaper- and even if you book a few days before- you get a discount that's worthwhile, and will have a better chance at predicting the weather.
My family uses the Internet to book, as we'll go whatever the weather, and so we find it very useful. However, some people should use this facility, as they are missing out on a trick. To be fair, not only the customer benefits (if the weather is good) but Merlin benefits- which gives them a higher profit, and so better rides can be built/maintained/fixed.
Therefore, online booking=good 8)