Lets start by telling you what a toner is. It is a term used for a colour, its semi permenant and dependent on the depth of the toner it will wash out of the hair gradually. It isn't as powerful as the colour applied dry when you are getting your roots or highlights done, it is a lot softer. It doesn't lift the hair or darken below 1 level darker.
Using a tonner doesn't mean the hairdresser has done something wrong in the colour process. It could be to create a white blonde which bleach alone doesn't do or to add a more golden glow. There are a few different reasons you would use a toner.
To remove unwanted tone; brassy blondes, unwanted wamth that bleach can leave
To add tone; if you want more gold, a bright red or soft pink gloss
If you have bleached the hair, we rarely ever leave bleach raw, it softens the out come and adds shine
To add shine
To refresh balayage