This is known as portfolio drift and if gone unchecked may result in you taking on more risk than intended, which could impact your returns.
For example, if you decide to have 70% of your money in stocks and 30% in bonds this could become 80% stocks to 20% if the stock market grows at a faster pace than bonds. This is known as portfolio drift and if gone unchecked may result in you taking on more risk than intended, which could impact your returns.