Blocking useful freedoms
Software patents block useful freedoms, such as the freedom to write software, to distribute software, and in some case, the freedom to talk about software.
Proof that these freedoms are useful
The freedom to manufacture cars is not very practical for individuals (see: software is different). In contrast, the freedom to develop and distribute software has lead to the creation of very large software projects including the GNU/Linux operating system and other free software.
Compatibility is essential
In software, access to data format specifications holds a role of importance unparalleled in other fields. Examples of data that come in specific formats include email, images, and word processing documents. In the context of writing an email reader, a word processor, or an image viewer, being blocked from reading, modifying, or writing in the required data format is equivalent to being banned from writing a functional program for that task.
They're freedoms everyone has
Software development, like writing a book or writing music, is something anyone can do. Most people will never write a book or any substantial amount of music, but some people do, and society is enriched by the work of those people. It's also good to know that if, one day, you really do want to write a book or some music, there's nothing stopping you. No one will send you a solicitor's letter claiming they have a storyline patent on an idea that you've illegally used in your book. Similarly, if you ever want to write software, for yourself or for others, at no cost or for a fee, you should be free to do so.