Strength is a fundamental skill that allows you to carry more items at a given time. Specific jobs rely heavily on your Strength level. Per level, you unlock an extra 10KG of weight. It is currently the only skill that is capped at level 30, meaning your max inventory capacity is 300KG (345KG with Premium). Using trailers and trunks you can go far beyond this limit. Trailers are also essential for advanced trucking routes.
Earning Strength Experience
Cargo-related jobs (such as Trucking and Cargo Pilot) rely on you having a big inventory capacity. Certain jobs and activities in Transport Tycoon will earn you Strength experience to be able to carry more items.
Trucking is the first skill most players come in contact with that also gives Strength experience.
Erasers small weight combined with its decent XP per item makes it the fastest method of obtaining Strength experience, compared to other Trucking Cargo.
Garbage Collector might not be the obvious choice when looking for a job to increase your Strength level, but it's worth it.
Cleaning up Los Santos will give you lots of XP in various skills, mostly Garbage Collection of course, but you will also earn decent XP in Trucking and Player skills.
The Cargo Pilot Jobs not only relies on Strength but also gives it to you.
Transport Air Cargo around the Map from Airport to Airport to gain Strength, Player and Cargo Pilot for Finishing your Route.
Besides trucking, most Specialized Jobs will also reward the player with Strength experience:
In the UPS Job you deliver packages to houses, warehouses, and stores.
Upon delivering you earn Strength, Player & Trucking experience. Finish the route for a bonus.
When repairing transformers you will earn Strength, Player & Mechanic experience.
As with the Transformer job, repairing servers will result in Strength, Player & Mechanic experience.
Delivering Coffee and Documents to IAA employees will give Strength & Player experience.
Mining at the quarry will give Strength & Mining experience.
Search around the Prospecting Field to find valuable Goods and earn yourself some money.
Along with Money and Goods, you also increase your Strength and Mining Levels.