The speech skill increase depends on the base value of the item traded or the difficulty of the bribe or persuasion.
The Speech skill allows you to more effectively persuade people and to barter with shopkeepers.
Perks in this skill allow you to get better deals when trading, buy and sell any kind of item to any merchant, and make it more likely for a persuasion or intimidation attempt to succeed.
Also i dont want to miss any of the story or any side quest, does using these skills make you skip stuff even if its not that important ?
Speech is a skill in The Elder Scrolls V: Skyrim and is one of the six skills that falls under The Thief playstyle.
The Merchant perk also allows you to buy any item from a merchant's inventory.