Now imagine someone comes along and says that in addition to your taxes, you have to pay $5000 to use this interstate.  This is called "barrier to entry".  Nobody but those who could afford the $5000 could use the interstate, the prices of whatever product was transported on the interstate would be higher, the cost to you for that product would be higher.  Everyone loses.

Now picture open source software.  Everyone benefits from its use, anyone can contribute, but no one is required to.  There is some cost associated with developing it, although not always.  There is almost no cost to distributing it.

Enter software patents.  Anyone, at any time and any price, can extract a toll from you for using, developing, selling, or distributing open source software.  To the detriment of everyone, except for the owner of the software patent.