The writer of the first quotation, "The road of excess leads to the palace of wisdom" might think this way because he or she has observed in others, or experienced themselves, that living a life of great excess (with great abandon) doesn't necessarily lead to happiness and fulfilment.
One can look at the story of the Prodigal Son in the Bible (New Testament), and how, in the end, he came to his senses and returned to his father - and family and friends - for forgiveness after living a life of excess. He saw the folly of his ways and gained great wisdom as to what is really important in life and turned back towards home to make things right and embrace a new start in life.
The writer of the second quotation, "Moderation in all things" might think this way because he or she has seen and/or experienced that not overdoing anything in life is often the way to enjoy blessings without abusing blessings. This can pertain to moderation in eating, drinking, working, recreation, study, sports, and more. For example, enjoying a nice single cup of morning coffee is drinking coffee in moderation. Having 7-10 cups of coffee during the day is overdoing it and one may reap undesirable results from this overindulgence.
The first quote alludes to the fact that we learn from our mistakes and experimentation; we end up understanding ourselves better, and hopefully we make changes for the better in our lives. The second quote alludes to the fact that we should not get carried away with a certain habit, act, desire, and such; but rather temper our behavior so we enjoy what we're doing without taking it to a level where we harm ourselves and/or others.
These quotes can be interpreted as such:
The road of excess can lead to knowledge that results in us living a life of moderation.