Q:

What is the purpose of the "Handy time stamps" in the time package?

Reference: 
I am trying to figure out what the "Handy Time Stamp" are used for in the time package. 
I can parse dates just fine using the other constants such as RFC
t, _ := time.Parse(time.RFC822, "02 Jan 06 15:04 MST")
fmt.Println(t.Unix())

Output 1136214240

vs 
t, _ := time.Parse(time.Stamp, "Jan _2 15:04:05")
fmt.Println(t.Unix())

Output: -62135596800

The last output is wrong. What am I missing here? How are these timestamps useful? 
Below is the Godoc for time constants:
const (
        ANSIC       = "Mon Jan _2 15:04:05 2006"
        UnixDate    = "Mon Jan _2 15:04:05 MST 2006"
        RubyDate    = "Mon Jan 02 15:04:05 -0700 2006"
        RFC822      = "02 Jan 06 15:04 MST"
        RFC822Z     = "02 Jan 06 15:04 -0700" // RFC822 with numeric zone
        RFC850      = "Monday, 02-Jan-06 15:04:05 MST"
        RFC1123     = "Mon, 02 Jan 2006 15:04:05 MST"
        RFC1123Z    = "Mon, 02 Jan 2006 15:04:05 -0700" // RFC1123 with numeric zone
        RFC3339     = "2006-01-02T15:04:05Z07:00"
        RFC3339Nano = "2006-01-02T15:04:05.999999999Z07:00"
        Kitchen     = "3:04PM"
        // Handy time stamps.
        Stamp      = "Jan _2 15:04:05"
        StampMilli = "Jan _2 15:04:05.000"
        StampMicro = "Jan _2 15:04:05.000000"
        StampNano  = "Jan _2 15:04:05.000000000"
)

A:

The last output is wrong. What am I missing here? How are these timestamps useful?

You are definitely missing an error check here, let's add it
t, err := time.Parse(time.Stamp, "Jan _2 15:04:05")
fmt.Println(err)
fmt.Println(t.Unix())

Output:

parsing time "Jan _2 15:04:05" as "Jan _2 15:04:05": cannot parse "_2
  15:04:05" as "_2"
  -62135596800

The correct string would be "Jan  2 15:04:05" (note the double space between Jan and 2). About underscore from the docs:

Within the format string, an underscore _ represents a space that may
  be replaced by a digit if the following number (a day) has two digits;
  for compatibility with fixed-width Unix time formats.

Then, why it's representations as UNIX time is negative, let's check:
t, err := time.Parse(time.Stamp, "Jan  2 15:04:05")
fmt.Println(err)
fmt.Println(t)

Output:
<nil>
0000-01-02 15:04:05 +0000 UTC

So it's negative because the year is 0000.
And finally, where it can be useful? For example, to measure duration of time-consuming operations. You can output to logs current time in one of Stamp formats along with some messages like "Started doing this", "Finished doing that". Then, because it's fixed-width format and without unnecessary year information - it's easy to read the logs, easy to parse such logs.
This format is actually used in "syslog" in *nix.

