Question
Why worship, respect and do pradikshna for Rahu and Ketu?
Answer
Because the scriptures ask us to worship them. That's why they are worthy of worship.
There is a detailed chapter (called Graha Shanti) in Yajnavalkya Smriti dedicated to Navagraha worship.
Verses from that chapter are given below:
CCXCV.—A person desirous of Sri or good fortune, or desirous of S^anti or propitiation should perform Graha Yajna (sacrifice to the planets). So also a person desirous of rain, long life and health. Similarly if he desires to perform sorcery also.—295. (Chapter 12)
So, the above are some reasons why one should worship Rahu, Ketu and the other Grahas.
Surya (The sun), Soma (the moon), Mahiputra (the son of the earth, Mars), Somaputra (the son of the moon. Mercury), Brihaspati (Jupiter), Sukra (Venus), Sanaichara (Saturn), Rahu (the ascending node) and Ketu (the descending node), these are remembered as Grahas.—296
Next the text gives the details of performing the Yajna/Puja. Like how to invoke them, what Samidhs, Mantras are to be used for each of them etc.
The fire-stick for the sun should be of Arka, for the moon Palasa, for the Mars Khadira, for the Mercury Apamarga, for the Jupiter Peepal, for the Venus Udumbara, for the Saturn S^ami, for the Rahu Durva, for the Ketu Kusa. These should respectively be the Samidhs for the respective planets.
Also, the primary reason we worship the Grahas, whether it is Rahu/Ketu or any other, is getting rid of their malefic effects. Because Navagrahas can not be Ishta Devatas.
Moreover, according to a boon given by Lord Brahma, the Navagrahas must worship those who worship them.
Whenever a planet has a bad aspect (in the horoscope of a person), that person should worship with great care specially that particular good planet. For Brahma has given this boon to them *' Being honoured you will honour him/'—307.
And, especially the kings of the states are particularly advised to appease the Navagrahas because they responsible for both the origin and destruction of the world. Because they control our destiny/fate.
By lords of men should be worshipped with greatest care the planets because the rise and fall of monarchs is dependent upon the planets, so also the origin and destruction of the worlds.—308.
Similarly, the Matsya Purana also details of how Navagraha Shanti is performed:
Karaala vadanah Khadgacharma Shuli Varapradah,
Neela simhaasanascha Rahuratra Prashasyatey/( Rahu Deva is fierce looking, carries sword, skin sheath, Shula and Vara Prada by his four hands and is seated on a blue throne).
Dhumraa Dwibaahavah Sarvey Gadino vikrutaananah,
Grudhraasana gataa nithyam Ketavah Syurvara pradaah/(Ketu Deva is of grey colour and of fierce Rupa with two hands showing gadaa and varada mudra seated always on a vulture)
Sarvey kireetinah kaaryaa Grahaa Lokahitaavahaah,
Hyaanguleynocchritaah Sarvey shatamashtottaram sadaa/(These are all beneficient Grahas worthy of ornamentation with Kiritas/head gears and be of hundred eight inches of Pratimas / Idols).
So, the scriptures themselves mention them as worthy of worship. And, in matters like religion, scriptures are our primary guide without which we can not do much on our own.
Also, note that prior to the Mohini episode, when Rahu and Ketu were not separated (by Lord Vishnu), the Asura was not worship-worthy. It's only after the head and the body of the Asura were separated they were included into the Navagraha group and regarded as worthy of worship.
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