Allah The Glorious teaches us in Quran to go to the Waseela first who is the Prophet (SAWA) to seek the prayers of the Waseela for us. ( Had they when they did wrong to themselves, come to you, and seeked forgiveness and got the prayers of the Messenger for their forgiveness, they would had found Allah Accepting repentance and Most Merciful) ( Sura An Nisa, Verse 64).
All our needs and wishes are in the hands of Allah, and He is The Giver. Waseela is not an assistant or partner of Allah but His most humble servants and He wanted us to be always connected to them to follow them in obeying and worshiping Allah, that is why we are been ordered to seek the Waseela to reach to real worshiping and real obedience of Allah (SWT).
Obviously, the Muslims should go to the successors of the Prophet (SAWA) after his demise.
We also supplicate to Allah ( The Glorious) directly but with mentioning our Waseela to Him as He ordered us. Mentioning the Waseela means that we are obeying Allah in our way of worshiping Him and confirming our allegiance to His most humble servants.
Our Waseela are the best servants of Allah and mentioning them means that we want to be good followers of them in worshiping Allah The Glorious.
This is according to The Real Tawheed of Allah, not as some people wrongly claim it away from Tawheed. In fact those people misunderstand Tawheed and mix between the due respect and worshiping. We respect and follow and seek the prayers of the Waseela as Allah ordered us, but worship Allah only.