I recently made some graphs in Illustrator CS6 to insert into a MS Word document. I created my original images using the Print profile with PPI set to 300. I used the "Save for Microsoft Office" option to export the images for Word. Why does this option create a PNG file (with a resolution of 150) as opposed to using, say, TIFF or EPSF format? I'm just curious.
Because that's the best, most reliable, format for Office.
If you're saving something for Office, chances are it is not going to be commercially printed. In that case, 150ppi is plenty for most home/office inkjet printers. In addition, only an RGB image is needed. Of the standard RGB formats, PNG24 supports transparency the best. So, 150ppi, RGB, transparent image = PNG24.
There's no call for a tiff or eps if using Office. The advantages of a TIF/EPS using CMYK color are pointless since Office doesn't use CMYK color. The advantages of an EPS being resolution independent are largely lost on the average Office user.