ולעולם ייזהר אדם מברכה שאינה צריכה, וירבה בברכות הצריכות; וכן דויד אומר “בכל יום, אברכך; ואהללה שמך, לעולם ועד” תהילים קמה,ב
The Rambam concludes the laws of Brachot with a tension underlying the entire area. On the one hand, we want to maximize saying Brachot. As a way of thinking about God every opportunity is valuable. On the other hand we cannot bless God willy-nilly since that would be disrespectful, instead we must wait for an event which calls us to say a Bracha.
This expresses itself with two opposing impulses, on the one hand extreme care not to make unnecessary Brachot, while maximizing the opportunities for a Bracha to be necessary.
There are two senses of unnecessary, and it seems that only one is a problem. The basic type is when the Bracha is not relevant. For example, randomly saying the Bracha of “Shehakol Niyeh Bidvaro”. While this Bracha is true at all times, we only say it when we eat something. All praise of God is for our benefit, and definitively incomplete, and therefore we need a reason to focus on a specific praise.
The other type is a situation which demands a Bracha, but which we could have avoided. For example, eating something in order to say a Bracha (which is the opposite of avoiding bread, so as not to need to wash and bentch). It seems to me that this type is not considered unnecessary, rather it is the method to increase needed Brachot, by increasing the situations of need.