This is a notational issue (and I agree with @SpineFeast's comment above). The book is using the way the number is written to give you information about how accurate the value is.
Writing $100$ means that our value could have been rounded. In other words, we did some calculation and got an answer between $50$ and $150$ and rounded our answer to the nearest hundred. In scientific notation, this might be written as $1.\times 10^2$ to indicate the number of significant digits. If one were to write $1.0\times 10^2$ this means that the we rounded to the first two digits, so the actual value is between $95$ and $105$.
Writing $100.$ means that we've put in a decimal point. The decimal point is unnecessary for the number $100$, but by putting in the extra work of the dot, we're indicating that everything to the left of the decimal point has not been rounded. This means that the value that we actually have is between $99.5$ and $100.5$. This case corresponds to $1.00\times 10^2$ in scientific notation.
Writing $100.0$ means that we're even more certain about the value. We've put in two extra things, the decimal point and the $0$ to the right of the decimal point. By writing extra, we're indicating how much more we know about the value. In this case, we might write, in scientific notation, $1.000\times 10^2$ to indicate that the actual value is between $99.95$ and $100.05$.
Why does all of this matter? The problem lies in that we sometimes round before being done with our calculations. Perhaps we want to compute
What does this mean, well, in a scientific situation, the $100$ and $30$ could both have been rounded before you received them, so the $100$ represents a number between $50$ and $150$ while the $30$ represents a number between $25$ and $35$. So, the actual sum could be anywhere between $75$ and $185$, a rather large range. The difficulty here is that the two numbers have been rounded to their leading digit but because the leading digit is in a different place value, the errors in each number get complicated.
If, on the other hand, one were to compute
then the decimal points mean that you're adding a number between $99.5$ and $100.5$ to a number between $29.5$ to $30.5$. The result is between $129$ and $131$, a much smaller range. So, the decimal point tells you that the values are more accurate, and, since they're more accurate, you can get a better range of possible values after operations.