Here’s a basic example of why the interpretive methods of Islamic Law (uṣūl al-fiqh) matter so much when looking at textual evidence.
Abū Hurayrah (may Allah be pleased with him) said that the Prophet ﷺ said, “Do not precede Ramadan by fasting one or two days – except for a man who was fasting a fast, as he is to fast it.”((Bukhari  and Muslim  transmitted it. Ibn Mulaqqin included it in Tuhfat al-Muhtaj , and Ibn Hajar included it in Bulugh al-Maram .))
What do “Do not precede” and “he is to fast it” indicate? Do they indicate obligations or recommendations? An intuitive reading of the hadith leaves most readers understanding that fasting those days is unlawful and that someone who is already fasting is obligated to fast them. This, however, is not exactly how many scholars – including the Shafiʿis – understand the second command.
First, some basic interpretive rules:
R1. The default for a command to perform something (e.g., “Do X!”) is that it indicates obligation.
R2. The default for a command to abstain from something (e.g., “Do not X!”) is that it indicates prohibition.
In both cases, there may be justifications for diverting from the default.
Following R2, the phrase “do not precede Ramadan by fasting one or two days,” is understood to indicate prohibition.
But what about the phrase “he is to fast it”? Based on R1, one expects it to indicate obligation. (And that’s what our intuitions suggest, too.) But there’s another interpretive rule that presents one of those justifications for diverting from the apparent default:
R3. The default for a command to perform an action after a command to abstain from that same act (“Do not do X!”…“Do X!”) is that it indicates permissibility (not an obligation).
One of the proofs for the soundness of this rule is that Pilgrims are prohibited from hunting while doing Ḥajj, and once they complete it they are then commanded to hunt.((Quran, http://quran.com/5/25:2.)) Even though this is a clear command to hunt – most scholars do not consider it to indicate an obligation to hunt.
So, using R2 and R3, the hadith is understood to indicate that it is unlawful to fast the two days before Ramadan, and someone who is already doing a habitual fast can go ahead and fast those two days – though he is not under an obligation to do so.
While R1 and R2 are something that intuitive readers typically follow, most will not have something like R3 in mind. This leaves readers with an understanding of the hadith that may be at odds with how the individuals who received Revelation understood it. And the absence of clear interpretive rules tends to also lead to inconsistent understanding. The example here concerned a matter internal to an individual hadith. Imagine what happens when one is trying to understand how several verses and hadiths fit together.
If you’re interested in this topic, be sure to check out Imam al-Juwaynī’s introduction to Islamic jurisprudence.