Biologists saw this as the natural means which they could copy, substituting other genes in place of the Agrobacterium sequences.
The reason why this is easier to do with plant cells has to do with the ability of the somatic cells, like those in the leaves or stem, to be reoriented in their developmental pathway to reform an entire plant. This means that if you introduce DNA into one cell, you can recreate a plant whose cells all contain the new genetic information. Today nearly all plant species can be manipulated as such.
The first attempts at gene manipulation made use of easily transformable varieties and easily identified genes such as those having to do with herbicide tolerance and pest resistance. Presently, plants are being grown commercially utilizing these approaches such as the Roundup Ready soybean, Bt corn (injected with Bacillus thuringiensis, a naturally occurring toxin used by organic farmers), and viral resistant papaya. Later, scientists grew more and more interested in using biotechnology for traits such as those that would permit plants to grow under foreign and/or adverse conditions, and those that would improve the nutritional quality of food.
The latter category would include foods with lowered allergenicity, like in the cases of wheat and peanuts. They could also be foods that contained a certain vitamin complex or even used to carry medicines such as edible vaccines for cholera. Plants could be used to make alternatives for products that are currently made from non-renewable resources such as industrial oils and plastics.
Despite it's positive potential, a portion of the public is nervous about the products biotech is creating and who is developing them. If constructive, however, such questions and discussions can help shape responsible deployment and use of the technology. Concerns have become increasingly contentious in the US following strong outcries by consumers in Europe.