The type for the children prop of EuiTooltip is React.ReactElement<any>. It expects this because it does a React.cloneElement to create the anchor.

Because it is of this type and not React.ReactNode, you cannot pass a string as children to a tooltip, so:

<EuiToolTip>{someValue}</EuiToolTip>

has to become:

<EuiToolTip><>{someValue}</></EuiTooltip>

It would be nice if we were more flexible and allowed any React.ReactNode as children.