The meaning of the word Caste, has been modified generation after generation resulting into a completely different perception in today's reality. It is not what we think of it.
When the creation blessed this planet with humans, they naturally started growing. They soon figured out that there are a certain set of tasks that everybody needs, on a daily basis for living. And they needs to be divided among different set of people otherwise it will become very difficult for each and everyone to do everything by himself.
So, they kind of had a meeting and depending upon the nature one possess they divided their set of duties. The basic nature of a person is nothing but the high and low level of different planetary energies within them.
A person with high influence of Jupiter, will be a Bhramin and can work as a Priest or Teacher. Strong Sun, Rahu and Mars people are the rulers, administrators and warriors. They are the Kshatriyas. High Mercury people are good accountants and merchants, so they all are clubbed as Vaishya. And the people who are naturally helping in nature and feels happy when they provide a sevice to others were categorized as Shudras. Humanity is bound to develop and this can not be possible if we make humans immortal. They need to die and they need to reproduce. The generation chain begun.
Due to DNA, similar traits were being passed on to the next generation and the influence of Karma was not too strong as the mankind had just begun. The kids were similar in nature to a greater extent and they naturally adopted the tasks their parents used to do. As, the environment we grow up is the source of all the data our brain feeds till the age of 12 as a User manual.
I will write more about it in some other post.
The same set of duties were getting passed for centuries and they started believing that blood is the basis of this categorization. And different caste refer to different group of families, that were recogonized due to the work they do.
In few more generations it became the default information. And due to the karmik influences people started changing their career paths based upon their interest. The son of every accountant is no more an accountant, they might be even a teacher or politician. A lot of new career options started emerging and the formula of passing the same profession to next generation was failed. A few more centuries passed and they only know that if someone is Aggarwal, his family name is Aggarwal. Assuming him to be a good merchant just because once someone is his family chain had a natural interest towards this profession does not make any sense if you read this post right.
Then why do we ask for a same caste person for marriage when there is no system and reason ? At the beginning an Aggarwal father would look an Aggarwal daughter for his son as she would have been raised in a similar living atmosphere. Also, she would be familiar with this profession and could help or support themselves as well. Above all she will feel homely and can understand their family situations better due to familiarity. It all started with a reason, not existent anymore !