A font swap can quietly wreck a design
If a presentation uses a distinctive font and the computer opening it does not have that font installed, the software silently substitutes something else -- which can throw off spacing, push text off a slide, or just make the whole deck look different than intended. Embedding the font solves this at the source, so the file carries its own fonts with it.
The two embedding options trade off file size against editability
Embedding only the characters actually used keeps the file size increase to a minimum, but if someone later types brand-new text on another computer, those new characters may render in a substitute font. Embedding every character makes the file noticeably larger, but guarantees the font renders correctly even for text added afterward.
Frequently Asked Questions
Does embedding fonts make the file a lot bigger?
It does grow the file by roughly the size of the font data, but choosing "Embed only the characters used" keeps that increase to a minimum since only the actually-used characters are included.
Can the embedded font still be edited on another computer?
If you saved with "Embed all characters," the recipient can edit text freely even without the font installed. With "Embed only the characters used," any newly typed text may show up in a different font.