With your WWF example, the purpose is clear, but there are many ways to try to get people to dig into their pockets. Do they tell success stories or sob stories. Many of these adverts try to draw us in and then bring up the subject of cash- look for the place in the text where they ask. Who are they trying to appeal to- young old, rich, poor, everyone, etc? Do they address the reader directly with words like 'you'? Which adjectives and adverbs do they use to try to make us sympathetic to their cause. Do they give facts and figures?
Does the text include images? If so, look at them too. After all, the WWF has used the panda as a symbol for years because it is so cute, even though many scientists believe it is an evolutionary dead-end.