I've recently been looking into movies, literature, music and art from the 50s-70s. During this period it seems that pin-up girl stickers, decals, posters, etc have rose to prominence within American culture; seemingly associated with WWII and G.I.s, for a reason unbeknownst to me. What's up? Why were they ubiquitous during that era?
I wrote my thesis on this in college (which, admittedly was a while ago, so forgive any errors or over-simplification). I love pin-ups!
Pin-ups, and pin-up posing/imagery was popular with both men and women during the war. For the troops it was twofold - a reminder of home and eventually a talisman for their mother/sister/daughter/lover who they are "fighting for" while overseas. For some women, pin-ups were symbols of productivity in the war effort (there are two great propaganda campaigns by Buell and Devorss trying to get women to join auxiliary groups).
The government used the popularity of pin-ups to work into their propaganda - there were "good" pin-ups that were mostly clothed and oftentimes wore military-style uniforms - these women encouraged troops and encouraged those at home to buy war bonds and participate in the war effort. There were the "bad" pin-ups who were more provocative in their posing and had less clothing. The Army would use the "bad" pin-ups posing or imagery in posters and pamphlets to warn soldiers away from loose women, VD, or espionage.
In the end, the popularity of pin-ups continued once the soldiers returned home and as the post-war nuclear family developed, the pin-ups became more overtly "female" by increasing the sexuality in the photos or illustrations. Eventually, pin-ups were isolated to mens' magazines like Penthouse - this development is most evident in Bill Ward's pin-ups. The women in the illustrations lost their agency and gained improbable bust/waist ratios.
I wrote a paper partially on this towards the end if my undergrad. Pin up models and the majority during the second world war what is a mix of morale boosting propaganda, a self reflection of the dire nature of the war itself, and a individualization affect for many units in the air and tank corps.
I think the most on-the-nose answer for this is obviously the morale boost pictures of this nature can have for young men missing home. pin-up models are also unique because they also show women and a multitude of poses but also costume. The second world war saw such large conscription and voluntary enlistment women were needed in the now vacant factories, hospitals, universities, offices, and many other male dominant industries. The constant letter-writing many American GI took part in kept them up-to-date with what their families and loved ones we're taking part in back home. This further acted as a boost in morale for the net on the ground reminding them how it wasn't just them being forced out of their normal roles and making sacrifices. The iconic poses and costumes of pin-up girl imagery play into the realistic fantasy of seeing women now occupying those ones male-only roles. Keep in mind, the mindset of a 1940's 17-34yr old. While for some men this may have been an erotic morale boost for others it was seen as a reminder of how they needed to return the world to normalcy so their loved ones didn't have to toil as they felt they did before conscription.
The other nite I can make on pin up imagery is how it was used has war paint on the side of many trucks tanks and military aircraft. Crews were often found naming their equipment after loved ones back home or made up names to rally spirit. This war paint acted as a morale boost for the operating men and can be found scattered throughout history books in many famous battles throughout the war. The most infamous of course being the Enola Gay dropping the first atomic bomb.