ーようにー can be used show when something is done to achieve an aim.
please speak slowly, so everybody can understand
We have learned that よう is used to show similarity, so this usage to show purpose needs some explaining:
In its simplest use よう（な）does indeed show similarity.
words that everyone can understand
words like those that everyone can understand,
words such as those that everyone can understand
The above phrase can put into a complete sentence.
Please use words that everybody can understand.
よう can also modify the verb directly, in which case it will take に because it is a な adjective
Please speak slowly, so everybody can understand.