I'm not aware of any particular GMRS frequencies being used for specific purposes but I'll point out that portable or mobile range is extended quite a bit through the use of repeaters and higher power, 5 and 50 watts for portable and mobile use respectivly.
Years ago, the 462.675 MHz primary was reserved for emergency and traveler assistance use. Many REACT groups across the country operated on this GMRS freq and many still do today.
This restriction on 675 was originally intended to be the GMRS equivalent of CB channel 9. Under the old GMRS rules, you picked any TWO of the GMRS primary channels including the repeater input freqs and listed the 4 freqs on the FCC license application. The form used was the old Form 574, the exact same form that was used for other land mobile radio services. The license issued by the FCC listed the 4 freqs you applied for plus a note at the bottom stated that additional freqs were authorized by rule section 95.29 and to refer to the rules for more information. This rule section granted you authorization under your GMRS license to use the 7 interstitial channels plus the 462.675 repeater pair for emergency and traveler assistance use if 675 wasn't already one of the primary freqs you applied for. So the smart thing to do was to apply for 2 GMRS primary freqs OTHER than 675 and you'll get the use of 675 by rule for emergency use.
With the creation of the Family Radio Service (FRS) in 1996, activity grew on the 7 GMRS interstitial freqs since the first 7 FRS freqs are shared with GMRS. FRS also used the 7 interstitial freqs adjacent to the GMRS repeater inputs. GMRS didn't use these upper freqs because of GMRS being an older radio service operated with wider bandwidth than FRS does and GMRS activity there would bother the repeaters. The increased activity on these freqs left GMRS licensees wanting more GMRS-only frequencies to use to be able to get away from local FRS adjacent channel splatter.
In 1999, due to the growth and popularity of FRS, the GMRS rules were amended to allow all-channel licensing where a GMRS licensee could use ALL 8 GMRS primary freqs just like they could under the old rules where licensees were limited to TWO primary freqs. The other thing that changed was that the restriction on 462.675 being reserved for emergency and traveler assistance use went away and 675 is no more "reserved" than any other GMRS channel is. The 22-channel GMRS/FRS "bubble pack" radios started appearing after this rule change.
is allowed to transmit with a mobile with 50W on the GMRS primary channels (through repeaters and simplex on the repeater outputs). The 7 GMRS interstitial freqs (same as FRS 1-7) are limited to 5W ERP. Mobiles can also operate on the interstitials with no more than 5W ERP. GMRS licensees can also operate through repeaters on the GMRS primary channels. Unlike the ham bands, GMRS licensees also need to secure permission from the repeater owners to use their repeaters whereas unauthorized use could be construed as harmful interference. You can have your own repeater setup on GMRS if you want to and you don't have to share its use with anyone if you don't want to. All licensees are still required to cooperate in the usage of channels, monitor before transmitting, yield to emergency traffic, etc..
Unlicensed "bubble pack" radio usage has become a big problem in GMRS. Most users don't have a clue about GMRS being a LICENSED radio service. They also have no clue (fortunately) about the full potential of GMRS (higher power, good commercial equipment, repeaters, etc.). These users are very casual users of 2-way radio. You get what you pay for and these radios really are nothing but cheap consumer CRAP. The radio manufactuers also downplay the licensing requirement ("FCC GMRS license required" is in VERY fine print). I think the FCC may someday do some form of de-licensing for GMRS to accomodate the bubble packs. Hopefully it will be license by rule for the bubble packs as they are today (22 channels, 2W or less power, antenna permanently attached, no repeater capability) but retains the license requirement for high power and repeater operation in GMRS. Local bubble pack activity can be a minor pain in the arse sometimes but I can easily talk over them with my good commercial radios I use on GMRS. The bubble packs aren't a huge problem provided they stay low powered and don't have any repeater capability.
I've been licensed on GMRS since 1992 and I've fully witnessed the changes in GMRS over the years including the bubble pack invasion.