Speaking of other obscure California state routes -- but keeping the discussion in the Valley -- it's interesting to note that after the West Valley segment of I-5 was completed circa 1972, Caltrans adopted/extended several localized routings to serve as connections. While that in itself is not all that uncommon, Caltrans tends to "stick to the plan", adopting routings that have been previously legislatively established and subsequently shown as dotted lines on the official state highway maps. Curiously, none of these routes were shown on previous route listings or maps prior to their adoption & signage. These are (1) the extension of 145 from 180 near Kerman south to 5 Points, then SW to I-5 (and CA 33) north of Coalinga (this is the infamous interchange where a I-5 traveler needs to have his/her windows all the way up to avoid the (mal)odor of the adjacent Harris Ranch stockyards!); (2) the establishment of CA 269 south of 5 Points, along a southern extension of the north-south portion of 145; this crosses I-5 and terminates at CA 33 in Avenal; and (3) CA 165, which serves as a southerly egress connector from I-5 to Los Banos, but which extends north of there to CA 99 at Turlock. In addition to these newly-established routes, 33 was removed from its original surface routing which crossed over I-5 a couple of times before striking out on Derrick Road toward Mendota and multiplexed with I-5 from Harris Ranch to Derrick Rd.
Since none of these routings -- or even the West Valley region in general -- were previously planned by Caltrans or its predecessor before these deployments and reroutings took place (all, IIRC, between 1975 and 1983), exploring the impetus for their establishment would be an interesting exercise. Likely scenario: local state legislators and/or county officials, realizing that the West Valley had always been given short shrift by Caltrans, saw the opening of I-5 through their midst as an opportunity to correct a longstanding situation of either oversight or neglect, and took action to manifest their concerns. Alternate scenario: since this Valley area is dominated by large-scale agribusiness, those various firms pressured Caltrans (directly or indirectly) to provide state-maintained facilities for their transport needs. Either way, the establishment and signage of these routes occurred within a relatively short timespan (at least, historically, for road projects in California!).