L3 Book 6 Buy and Hold Approach in Micro Attribution

Book 6 中  Micro Attribution 关于buy-and-hold approach有这么一段话，求高手指点一下它的这个Advantage & Disavantage怎么理解的呢？

The buy-and-hold approach, which disregards the impact of transactions during the evaluation period, has an important practical advantage: Only the holdings and their returns need be input to the attribution system. There is, however, a disadvantage: The account’s buy-and-hold return will not equal its time-weighted total return.