Why did he feel that it was appropriate to start philosophy by examining the immediate sensual certainty?
The Phenomenology of Spirit is at least according to Hegel a preparatory work that is not a part of Encyclopedic system of philosophy (which he explains in detail in two different versions). The system is supposed to explain everything in all of its details and with a certain type of necessity (Hegel is coincidentally an originator of the idea of historical progress towards a necessary future).
I'm not sure how much justice I can do to "appropriate", but Hegel follows a basic methodological pattern just about everywhere (you can find one version of how to describe it in Taylor, Hegel, 1975, you can find others in Lauer's Hegel's Idea of Philosophy (1983)).
The basic pattern runs something like this:
- Take an idea or approach
- Present it with a problem case or resolve a built-in tension in the idea or approach.
- Amend or replace the idea based on two.
Hegel calls this Aufhebung which is a verbal noun from aufheben, which has as its English cognates Upheaval and upheave, but since in nearly all cases that turns the sentence into gibberish in English, it is translated as "sublation", a word which was coined for this purpose.
In general, but not always, step 1 at the beginning of any topic is supposed to be ordinary experience in its most immediate form. This then will need to be mediated to reveal its truth.
Okay all of that is background really.
The Phenomenology of Spirit is meant to be a study that looks at the nature of Spirit; it is simultaneously a study of the theory of knowledge. This makes better sense if you understand what Hegel means by Spirit, but it's hard to explain that without knowing the end game of the book to begin with. (In the briefest possible definition, Spirit = thought thinking itself but no longer Aristotle's quarantined God but instead an entire universe that thinks about itself -- with the thinking part being rational beings in it).
Roughly speaking the book progresses through the following stages:
- What does it mean to know something? (consciousness)
- What does it mean that I know something? (self-consciousness)
- What is this faculty that is asking these questions? (reason)
- What has this sort of reason? (Spirit)
- What is the truth? (Religion)
- What is the truth in all of its fullness? (Absolute Knowing)
Given that this is the basic road map, sense certainty is the antithesis of this, because (in the formulation Hegel gives it the first time it comes up) it lacks any sense of itself as conscious and is barely even a claim to knowledge.
At its most primitive, it's the idea that knowledge is just out there waiting to be perceived. It then has to differentiate itself into ideas about the permanence of the world and the existence and truth of things that we don't see. All of this propaedeutic to any thought of a knower or any more elaborate system of knowledge.
Thus, Hegel uses this as the start of his project about Spirit.
I am not quite sure if that addresses "appropriate" but that's at least how starting the book in this way fits into the project of the book.