Linking cause and effect can be difficult on your own, and over time a gap between feel and real can cause what I call "anti-learning". When a golfers response to shots is based on un-verified feels not facts, the fixes cause more harm than good.
Intrinsic feedback (or feel, see Awareness section) needs at least occasional verification. This section specifically deals with extrinsic feedback, i.e. coming from an external source.
Extrinsic feedback informs feel
Your physical feel for the swing can be educated and informed by external feedback. For example, if you are working on getting a later low point in your iron swing, being able to quantify this on each shot can help make more subtle distinctions in your feel for what you are aiming to produce.
Source of extrinsic feedback
Eye-balling the ball flight/roll
A coach's observation
Launch monitor data
Without going into each one in this article, obviously the last three are quantifiable sources of feedback. The launch monitor is the most important of the three because it is recording the effect you create on club and ball. The only absolutes.
If there's one thing as bad as no feedback, is too much or irrelevant feedback. Unsolicited, irrelevant observations from your playing partners "Your head moved" is likely to harm more than help. Here are some guidelines to ensure feedback is helpful.
What's the intent?
The most important question to answer is, how did you do in relation to your intention?
You can't assess a ball flight properly if you weren't clear what the intended shot was. If you flush one out of the middle, starting straight with no curve, it's not a clean sweep of all the impact goals, if you actually meant to hit a soft draw. Whilst the straight shot may have been enjoyable, there is limited learning if you're not making a distinction against intent.
The same goes for movement. If you are working on improving the balance in your swing for example, you need to establish clearly what that means (e.g. finishing with your weight round near you left heel) so you can make sense of the feedback in your next attempt. "Did I make a good swing?, is too broad a question in this case.
Focus the area of interest
The feedback floodgates will burst open and overwhelm you if your area of interest is too broad or vague. Taking the draw intent as an example, one way of filtering the feedback is to simply ask one or both of these questions:
1) Did it launch right of target?
2) Did it curve left? (n.b. this is for a right handed player).
A nice simple brief, that gives you clear, actionable feedback.
Learning is a constant cycle of awareness and correction, and since we are only aware of what we are aware, external feedback fills the gaps.